In theory it’s easy to monetise - allow some targeted ads to communities and/or occasional relevant boosted posts, or paid awards like Reddit, etc.

The issue is greed / growth. They always need more and more - so you end up with more irrelevant ads, political ads, more boosted posts than natural ones, etc. - most companies aren’t happy to just do one thing well with a skeleton crew maintaining it and keeping costs low - they need constant growth.

Just look at Reddit and Twitter for example.