Why does the open source community use GitHub, a closed source platform?

I wouldn't speak of an "open source community". there is no such entity, formally speaking.

the best reason however is, in my opinion, network effect. github has been the first big player in this space and is also the default place where people go.

However yes, I think that the open source community should move to at least gitlab.com, if not to sourcehut.org.

The open source community is a vast and non-uniform group, so there’s not going to be any unified reason. There are also plenty of people (both within the “open source community” and unrelated to it) who don’t use GitHub, likewise for a variety of reasons.

But speaking personally, I’m totally okay using closed source services and tools when they are a good fit for what I need. The fact that I work on open source doesn’t imply that I must refuse closed source services.

I think this is an instance of a fairly common pattern.

Not all good ideas/implementations arise from the FOSS community. And sometimes, there's something that's so clearly better than the alternatives, that it becomes widely used despite being commercial.

Previous examples I'd cite are Solaris, Purify, vTune, BitKeeper, Sublime Text, macOS, etc.

Generally, some time passes until it becomes clear what it is exactly that is key to the experience, and those things are then either replicated or extended in FOSS, and people then move away.

eg. Linux has replaced Solaris, Git replaced BitKeeper, valgrind replaced Purify, etc.

There's also a spectrum of people in the community: some who are willing to use commercial products pragmatically, and others who aren't on a more purist stance. GitHub clearly has very broad adoption, but since the Microsoft acquisition especially, GitHub and SourceHut appear to be attracting more patronage.