Problem is, not everyone can host everything on their own server. I looked at Gogs, and the resources it need (if you need to use any team functionality) require a middle to high end VPS.

So, if you need to run 2-3 self-hosted services (Let's say, your own Gogs, MediaGoblin, Pump/Diaspora) it becomes very expensive to maintain. Though, I do agree with the points mentioned, but sometimes it's better (easier, cheaper, more reliabled) to go for a paid service then self-host.