I agree completely that you don't have to use them. The thing is though that many people DO like to use them. Clearly, because they are selling these splat books and people keep buying them. So I think many people want to embrace the entirety of the game. The bloat issue also occurs for new players. They are ignorant about tabletop stuff and when you see the gigantic massive amount of books and rules it can be a deterrent. Sure they can read up and study about what you need or what you don't need but realistically first impressions are hard to combat. Also, IF a new player does want to go and study up on what they might want or need the ever growing option list means more studying and learning.
System bloat is a thing, pretty much everyone agrees that 3.5 was god awful in this respect (the sheer amount of books being made, not the quality of the books or rules). Pathfinder clearly took a step back focusing more on their stories. This is great, like I said 4 books a year was much much better than 12 books that wizards were putting out each year (one per month ideally). However the longer time marches on, the bloat inevitably builds up. It's unavoidable unless you put out the 3 core books and leave it be. So now, as we are seeing with 5e, wizards has learned from the past and they are being VERY slow with their spat release. I think this is best for the game, but its own problems arise. People start feeling like the system isn't being supported blah blah.
Basically you are damned if you do and damned if you don't. Also don't get me wrong. I love pathfinder, I love splat books. I also like using lots of fiddly rules to change the feel of the game. Depending on what I want to use, it can be daunting if lots of splats are used. That's also the bloat problem. It's not that every option is being use all the time. It's that more rules are available, so you may only want to pick a few things from a few books, but then you have to keep up with all the books being used etc etc.
At least that's my take on it. It's a double edged sword. You can run a very detail oriented game using lots of little subsystems but it becomes harder to manage or you can run a rules light system (along the lines of 13th age) and miss out on all that detail that others might want. There's no right answer, and each argument is valid. The point I just wanted to make was that, if you step back and look at pathfinder as a whole. It's so incredibly complex. That's a wonderful and terrifying thing depending on your point of view.
