Because everything anyone says nowadays has a name above it, along with a number that is supposed to indicate the quality of the thing being said, or more like the amount of people who have read and agreed with the thing. That makes it seem like that thought is then somehow intellectual property of the person, while most of the time it's too trivial to have any sort of copyright law surrounding the thing. Yet people still want to enforce ownership, because it makes them feel good, and like it's them that contributed something to the public discourse and no one else. Ego and the need for social validation, which is something you can apply to almost every situation on social media that makes you question why someone said something. I must say I'm feeling kinda correct here, this may get validated and produce good feels for my primate brain

Thays a good question. Why do so many people do it?

You've actually defined - intentionally or not - what's left of the little bit on the internet once you've filtered out the five giant site orgy:

Trivial Bullshit