You don't need CUDA for gaming but software is still just as big of a moat. Gaming GPU drivers are complex and have tons of game-specific patches.

With their new Radeon/RDNA architecture it took AMD years to overcome their reputation for having shitty drivers on their consumer GPUs (and that reputation was indeed deserved early on). And I bet if you go read GPU discussion online today you'll still find people who avoid AMD because of drivers.

That won't stop them, but it's a big barrier to entry.

Oh and that's just to get the drivers to work. Not including company-specific features that need to be integrated by the game devs into their game codebase, like DLSS / FrameGen and FSR. And in the past there was other Nvidia/AMD-specific stuff like PhysX, hair rendering, etc.