Most of the time it's because client-side anti-cheat is not being well or at all supported on Linux.
For instance Roblox used to run on Linux through launchers such as Grapejuice or Vinegar. However the devs now explicitely block Roblox player on Linux due to a large quantity of cheaters coming from there according to their stats. Since their actual Linux userbase is minuscule compared to other platforms, they just chose go the easy (and probably financially sane) route.
As for Fortnite, it's all about the ROI baby !
If we only had a few more programmers. It’s the Linux problem. I love the Steam Deck hardware. Valve has done an amazing job there; I wish they would get to tens of millions of users, at which point it would actually make sense to support it.
Driving manual is not particularly complicated, but it will require you to focus a lot of attention on it at first. Since you also have to cope with with differences in driving regulations, this will put you and your passengers more at risk than necessary.
It takes time (~tens of hours) to build the muscle memory so that it becomes natural. For a vacation in the short term, you should go ahead with your plan to rent an automatic.