Because being user-friendly is way, way, way more important than things like privacy, not being bombarded by ads,

People gravitate to the technology that they can use.

I live in a small town. Every business and service has a Facebook group because it's usable. A couple of stalwarts maintain web pages but they're full of inaccurate information because they're too hard to update. The Facebook groups, on the other hand, are constantly up-to-date, and the owners are quick to respond to any comment on them.

That's before you get to network effects. Everyone in the town uses Facebook. So when kids ask for a messaging or social media account, they ask for a Facebook product so they can talk to their friends and family.

It's incredibly shitty. Everyone in town would agree with your sentiment, but they bounce off competing products. The effort involved in hosting a service isn't a higher perceived cost than the ads, privacy invasion, etc.