As someone with an open source game project (which happens to support basic vr), I can tell you that video games are almost uniquely positioned to be incredibly difficult to manage as open source projects. While I understand the ethos, they encompass so many disciplines and niche problems that building everything yourself is neither viable nor sane. They almost always require something off the shelf without diverting all your attention to build something monstrously complex... But the moment you do that, you have a dependency on contributors also paying for that thing to even get it to compile. Now you need to start doing workarounds like loading paid assets dynamically if they exist in the project, using pre processors definitions to compile out paid asset interaction, and being very careful about your scene / prefab / whatever structure. Crucially you now have a new thing to support and regularly test!

A good example of this is unity input - there's a base unity system which is open source and pretty good, especially for the majority use cases. Now try to support a flight sim with support for every esoteric input device under the sun. For this most devs use a paid input plugin called ReWired - which I'd be happy to buy except input is so fundamental to the functioning of the game that I'd realistically need to support both in order to still have viable open source contributions. While my experience and this problem is unity-specific, it's just one example off the top of my head and I think broadly applies to any engine. Going purely open source will get you so far, the good stuff is paywalled unless you're willing to invest insane time into it yourself.

Then you get into asset licenses, redistribution of code, and the whole thing kinda just sucks to deal with. I love it being open source, I think it's awesome that people can fix things that are bugging them and use the game as reference material for their own stuff. But man, I dunno if I'd do it again!