Because it's normal for humans as I see it to be selfish, and we've built systems that force selfish desires to clash repeatedly.
Selfishness has a bad reputation, but in fact it's how you end up not starving and broke in the street and able to keep a safe life. Pure altruism taken to a logical extreme would be dangerous, so there's always a balance of self-preservation based on your own wants and needs.
When you go to any public forum, nothing is really different; you do the things you desire and that protect yourself, you talk about the things you're comfortable about, you can most effectively comment about your experience and how items relate to you.
Where the problem with the above comes out is when this aspect gets out of alignment as well, as pure selfishness is just as disastrous. At a certain point to obtain your desires, you need _something_ from someone else, and likely that gets forgotten on social media that without the someone else, there is no social media. All other things in life require the contribution of human labor somewhere, even with more reliance on automation (someone's gotta maintain those machines and the code, even Star Trek still had an Engineering staff running around fixing replicators that made bad coffee/tea). By forgetting this, people feel entitled to having it exactly as they want, and why not? It's their leisure time, shouldn't it be exactly what they want to relax?
I absolutely understand the author's point, but in a sense, they're doing the exact same thing, and trying to define a requirement for social media that not necessarily everyone subscribes to. I happen to think this requirement is a good one, but not the point I suppose. Once you allow public access, you have to accept also the bad, and maybe even revel in it a bit.
And let's not forget how algorithms exacerbate this issue.
Older "anything-goes" simple forums appeal to me mostly because you get exposure to things you normally wouldn't and there's no feed that gets polluted with bad data if you just check it out and there's no rewards system to push the content up; you just have some entries in your browser history. The users that do try to make the forums "their own" get the option to do so without having a feed/rewards system push this data heavily, and users have an easier time avoiding such content. There's a safety valve for their desire without some system forcing their desire on others.
Feeds/rewards systems really muck up the situation because exploring a topic you aren't interested in or even dislike comes back to bite you in the ass because feeds aren't very good at getting when you're just checking something out versus when you're interested; any interaction is a change to induce further engagement so the feed tries. Rewards systems that promote content do much the same, but the system is powered by humans (and bots) instead.