They don’t care about FOSS, but they care about “computer lets me do what I want”.

Discord is obviously proprietary but it’s actually a very modular platform that gives a lot of nice controls. It’s easy to make your own “server”, it’s easy to add whatever bots you want, it’s easy to moderate. From a consumer perspective, it’s “open”.

Also, I know that this wasn’t your point, but I do feel compelled to point out that Discord works fine on Linux.

Right, but that proves nothing, is there something that is more open and better than Discord, for this group of people? Otherwise I'd say my argument applies in exactly the same way. Pragmatism wins, so why change unless there is a need?

Yes if gamers truly cared about openness and absence of corporate control, they would move to self-hosted Matrix channels.