My problem with splatbooks isn't with regards to the existence of the books themselves; I think most people are just fine with the existence of books that they don't like and don't want to buy (though to be fair, there are plenty of people out there who do seem to hate the thought of the things they don't like so much as existing).
Rather, my problem is that splatbooks put the lie to what I feel is the inherent promise of a role-playing game: that you can be anything.
Now, admittedly, this promise often comes with unspoken caveats, such as "...within the context of this setting that we're designing for," or "...within a certain scope of power that the game is built to handle." That said, there's still an expectation in a lot of games - particularly where there's an understanding that the game is meant to be universal in scope (which is what D&D has been for quite some time now) - that picking up the game will give you everything you need to play whatever sort of game (and, more specifically, whatever sort of character) you want.
Except then we find out that that's not true. The very existence of a supplement with additional rules carries the implication that the options it presents aren't to be found within the basic rules, and that we need to pick up this new book to fulfill the promise of being able to do truly anything in the game.
...until the next book comes out, that is.
It doesn't take long to realize when this becomes a cycle. The spectre of "this time, you'll have all the options" is a carrot that we're paying for to get yanked out of reach again and again and again. Worse, it's not hard to realize that the companies that produce these materials are incentivized to make sure that they never live up to that promise, since that would cut off an obvious revenue stream.
Splatbooks are the DLC of table-top role-playing games, and the objections that they draw are the same as the objections to those. People don't see them as new options to expand a game that's already complete; but as unending additions to an incomplete game that should have been complete out of the box.