Lots of female names have unstressed feminising suffixes (like -a) or diminutive ones (like -y/ie). They make the name sound softer. Think of masculine vs. feminine line endings in poetry (final syllable stressed vs. unstressed). Calling your sassy, tough-girl, martial-arts-wielding heroine "Sally", "Lucy" or "Amanda" doesn't work as well if you're trying to create a character like Kate Beckett.

Of course, there are other monosyllabic female names like "Ann", "Sue", "Rose" and "Eve". But "Kate" sounds harder than all of those because all its consonants are plosives (sounds you can't say continuously). And yet it's not an unusual name, so we can all think of a "Kate" who we know and don't dislike too much. That's my theory.