Very predictable: If Facebook (or any other social media site) showed you what you wanted to see --- stuff from your friends --- you would be satisfied and leave.

... but Facebook makes money off ads. They don't want you leaving. They want you to stay online all day.

Instead, they show you brainrot: content interesting enough to keep you on the site, but shallow enough that you are always thirsty for more. However, making this content is still a lot of work, and isn't what most people want to do: It takes a lot of brainrot to keep you trapped 24/7.

Slop requires no effort, costs next to nothing, and fills the "brainrot" niche perfectly. Facebook doesn't care that people are posting bot content, because it's the perfect thing to make them money.