That it's not about the consumption of brains, but they bite as a way to pass the virus from person to person. Yet, that's Neo-Zombieism, explaining away any other method of the undead with the idea that the body is merely a host to a virus which reverts a human back to a primal state.

Thankfully, some still adhere to the idea that a true zombie is a slow, shuffling, gaping and bemoaning zombie. Not some trussed up, Usain Boltesque zombie that could outmatch a cheetah. And if indeed, the zombie is merely a host to a virus which reverts a body to a "rage" state, after a mere lapse of time from initial infection, can they truly be considered undead? Are they not just alive, yet critically infected?

But, I'm not here to critique the recently altered trope. I'm here to tackle this question head on:

First, any living person whose brain is destroyed ceases to function normally. In fact, any living creature whose brain is destroyed is also out of luck. Unless it has no discernible anatomy, in which case you can't roll a critical against it anyway.

But as a particularly resilient species, humans can recover from a variety of other means of bodily destruction. And as zombies, this resiliency seems magnified. Zombies can survive indefinitely without a heart, and likely any circulatory system what so ever (this has yet to be field tested. In fact, our scientists never seem to return from said field.) Humans can survive what would otherwise prove fatal for many other species, such as bone breaks. When a horse breaks its leg, it's done for. Grab your empty glue pots and get ready, because Chestnut is about to be dog food. But when some idiot neighbor kid decides to climb your tree in your own yard, then is stupid enough to fall from said tree, and little Spencer's parents sue you for the medical costs, we don't take the kid out back and give him a mercy killing (regardless of how much Spencer may or may not deserve it.)

Yet, regardless of a zombies hyper-resiliency, it still suffers from the weakness in its cranium. Dismemberment, sudden onset lead poisoning, or a nice sharp whack, all prove fatal to living and undead. It may be hypothesized that, as the brain seems to be the great equalizer between the two, that it is the one body part to which the undead can still adhere any meaning. Further, in their instinctual state, zombies may misunderstand that their one weakness is, ultimately, the ONLY weakness there is and assume that to be true for all other lifeforms.

Of course, the "why" would depend on your preference of how Zombieism arises in the first place.

• If you were to take a purely biological view (disregarding the virology theory currently floating around various media), you could state that the brain represents a high source of nutrients. They are rich in cholesterol[citation needed], which is necessary in retaining cell membrane pliability[citation needed], a natural problem for cold-blooded (or no-blooded) individuals[citation needed]. Brains, once you colloquially crack the nut, require minimal effort to consume. It's soft, and squishy, compared to the rest of the flesh readily available[just straight up fact, yo]. Also, recent studies have shown that the majority of brain cells are akin to stem cells[again, citation needed], and by consuming brains, they are in essence attempting to restore their bodies to a living state. But, the brain is also the location of dopamine production, so they may just be trying to get high. The Gray Matter Platter, as they say on the street[as they also say "check your facts before you act, foo!"].

• If your zombies arise from some evil curse (hoodoo, demonic ritual, initiating a subprime mortgage), then the consumption of the brain can itself be viewed as a highly unholy act, the utter desecration of the one attribute we humans hold ourselves above other species. The act may even be the salvation against the curse itself. Consuming brains may be an instinct to restore the consciousness (or dare I say, the soul) they lack under their current malaise. Proper research into this matter proves ultimately fruitless, as whereupon the scientist contacts a demon, he/she is invariably given a formula for a better boner pill, promised millions of dollars in residuals, and dies soon thereafter in some tragic yet fittingly ironic method that only a demon would find humorous.

• If literary criticism is your bag, you could speculate that the consumption of the brain is a sociopolitical statement meant to evoke the notion that the zombies represent an unthinking majority of humanity. Zombies could represent mindless individuals who blindly accept what they are told, do little to expand their thinking beyond which Instagram filter to use on their picture of a bowl of cereal, and generally remain within the strict limitations of their life as defined by the guide on their DVR. In other words, "Sheeple." Therefore, eating brains is a method in which the zombie either attempts to gain something that they lack, or to subvert the smarter individuals around them in an effort to remove the perceived threat of intelligence.

But the method and reason to a zombie uprising isn't what we're here to answer. What are we here to answer?

Ah, yes. Right. Thank you. We're here to address how zombies continue to proliferate.

When a zombie bites a human, the period between initial contact and full conversion to undead varies widely between numerous reports. An attempt to solidify research in this area fails, as when test subjects are informed of our experiment (as is their right), they rapidly and vehemently unvolunteer from the experiment. Some even go so far as to reimburse our scientists for their time, and urgently insist that the scientists instead focus on a different field of study, typically that of committing rude intercourse upon one's self.

Full conversion may take minutes, or hours. And the difference may be vital in understanding Zombieism.

Imagine Zombie 1 (Z1) approaches Human 1 (H1). After typical chicanery, Z1 bites H1. H1 starts the transformation into a zombie. Meanwhile, Z1 begins to break open the skull to access the brain of H1, not exactly a quick process.

Now, if the conversion time is mere minutes, Z1 would simply not have enough time to retrieve brains from H1 before H1 became Z2. Once the conversion completes, a zombie becomes uninterested in another zombie, gastronomically. So, Z1 and its attempts are thwarted, and Z1 and Z2 go off to search for more food.

If the conversion time is, say, an hour, Z1 would have more time to break into the skull, retrieve the brain, consume, and move on, leaving the corpse of H1 in a grotesque mess on the ground.

And so, if H1 is without a brain—which can be assumed is just as fatal as a shot, dismembered, or bludgeoned brain—then H1 will remain H1, albeit sticky and truly dead. The complete consumption of the brain robs the conversion from fulfilling its course, and stops it just before it lurches to the finish line into undead territory.

Again, conversion times vary with any given media, and a multitude of other factors may contribute to the proliferation of Zombieism. A human may be bit, but escape being consumed, only to let the conversion adhere and begin the process anew (typically when it is least expected and typically in a place considered safe from any zombie outbreak, [i.e. military base, heavily modified safe-house, a sea-faring vessel, etc.]). In these unknowable situations, Zombieism can arise and prolong the spread as it continues to flare up through various means of transference.

However, I believe a zombies focus on the brain is intentional, and can be pared down to the simple idea of population control. If Z1 bites H1, and can consume the brains of H1 before conversion completes, that is one less undead Z1 needs to compete with to acquire more brains. It is a brutal method, yet one that results in a zero-sum game. The gains of Z1 (nutrients, non-competition from recent meal) balance the loss of H1 (mainly, death).

Much more study and testing is required before any theories can be postulated. There is much research needed in a variety of fields and sub-fields of study to, my apologies, to flesh out our understanding of the undead.

But until then, happy hunting!