Strong network effects + capitalism. They found a good way to make lots of money, which allowed them to pay lots of skilled people to spend their time getting more people into the system and looking at ads.

The problem, IMO, is that most users don’t understand how social media works from a business perspective. Advertising companies put a lot of effort into not being known as advertising companies.

Another fun thought experiment: consider who runs political ads, especially local advertisers (highway billboards etc) and what speech they’re willing to accept. Consider Comcast / cable networks and what speech they’re willing to accept. Who are their customers, how are they making money, and how does that affect what speech they are willing to accept on their platforms.