Because of the stupid shit people post there, including politics.
Ah, politics. I see, it can be really annoying. A couple of random people tried to add me as a friend and I accepted at first, but in both cases these people were posting a lot of polarizing political content and I'm pretty sure those are people with an agenda, so I can see how Facebook can become problematic like that, especially if someone you know IRL starts to repost this BS.
No one I know in real life posts anything political on their feeds though, thankfully. Maybe that's rather unusual, idk.
Really though, it's mostly the performative nature of it. Sad or glad, every post is dramatized to trigger a reaction. Maybe I just have bad people in my life, but I've heard I'm far from alone in my assessment.
Well, I just scrolled through my feed for a really long time just for a test and all I found were a couple of family pictures, a bunch of pictures of pets, alcohol and drunk shenanigans and the rest was just memes (I even ran into a suicide joke) and random posts like "oh, this exotic location sure looks pretty" and "oh, I'm so nostalgic for this bunch of TV ads from the 90s do you remember these too you old fart?". I suppose I'm officialy the most dramatic person I know, because no one else seems to post anything dramatized. But maybe that's just my social circle, if other people's experience is bickering over social and political stuff with people you know and constant dramatic grabs for attention, then I can understand why they don't like facebook.