I remember when you could go on Facebook and look through your feed at what your friends are saying, catch up with them, and browse posts that they have made. Now, it's just completely random and chaotic, almost nonsensical. There's no logical sense to my Facebook feed at all. As you can see in the image, they are showing me stuff that I'm not even following. This is not even something that I am actively a part of! It's some random group. So what's the point of following a group or liking a page, if they're just going to show you random stuff anyway?
Like, wtf happened to this website?
They figured out that this kind of feed makes the most money from the most users. They don't care if you like it, they care what the majority of users will stick around for. The longer total scrolling time they can get from their user base, the more ads they can cram in there. Ads make money.
Algorithm leads to more scroll time per person leads to more ads per user leads to more income
So, because money
I suspect it's not an optimization to make every post you see interesting. For one thing, we tend to find intermittent rewards more fascinating and addictive than reliable ones. For another, if you have to scroll further you'll see more ads. But if you make it too boring people won't scroll at all. So the algorithm probably tries to make it just interesting enough to keep people scrolling, but no more.