They use a lot of open source libraries, yes, but I think it's about how much of the end product depends on the OSS tool/library. Studios using unreal engine probably don't use that much critical OSS directly – their licensed software probably does. And the software vendors / big studios do donate to tools they depend on, for instance Epic Games donated $1.2m to Blender
AAA usually goes with AAA-tools and frameworks. So Unity, Unreal Engine or even their own engine. OSS might be used, but for smaller parts or as tools for producing their stuff (like browser, editor, etc.). So while they sometimes donate, there is not much reason to give a big sum to a single project. They might be even donating more overall, but separated on multiple different projects.
AAA studios don't really use MonoGame.