because you need to be clear as to whom you're talking about. If you're not, you either confuse your readers, or end up using many more words just to clarify something that could've been made clearer by using more unique names. I don't know much about professional editing, but I have a feeling that many professional editors would strongly suggest changing similar or repeated names.

Has anyone ever heard of Chung Kuo? Like RJ's work and "War and Peace," it has hundreds of characters.

I watched Lost, and I definitely get your point - there would be 4 Johnathans at least, and several Michaels... But they obviously named one of the big leads "Jack" so they could have another big lead named "John." No one ever calls them by their possible nicknames or full names ("Johnathan" for both).

As a story writer, you end up using a certain amount of economy to avoid confusion. So you name folks "junior" or "dad" because it helps bigtime when they bear the same name as their relatives.

I don't know how it works in other languages, but in Spanish, you can run into some real issues in terms of which person you're actually referring to - and those problems stem from pronouns, largely. It gets way more confusing when you're reading a narrative and people have the exact same first name. "Michael attacked you?!?! Do you mean the Michael that has a little boy and screams 'Waaaaaaaaaaalt!' all the time, or the Michael with a small pet-grooming business in Fresno?"