They're doing ports to Xbone and Switch and PS5 already, and that's fine. But running on Linux is somehow insane?
No. Porting engines across complicated SDKs and build environments is what these studios do every day of their existence, and virtually all the backends run fine on Linux already.
Again, the reason the big studios aren't on board is because they don't want to be part of an app store ecosystem. It's not a technical thing at all.
Depending on the engine it can be quite a lot of time. Especially if you don’t already have a Vulkan backend.
Therefore if you don’t think the additional sales are worth the expense, including the opportunity cost of your devs, why bother?
Ports to Xbox, Switch and PS5 have a long established record of making money, those devices have huge established userbases and they have very good toolsets for doing the ports. The engines will have already been ported.
None of those are true for Linux, and the toolsets for doing so are not mature in the slightest, for a completely unproven market.