>>404986589
In short, increased complexity and losing focus to gimmicks.
Compared to a flat game, where a lot of things are animation driven and you can get away with a lot of shortcuts, VR is much more detail focused. That small thing that would never get close to the camera now can be picked up and examined closely. That part of the object that would be normally hidden could be rotated in the player's hand now. A swing that was a simple animation and collision check now need to check velocity and controller position.

There's also a fact that with VR, immersion becomes big. If you look at any VR Dev tutorials, most of them about gimmicks like hand animation, physics interaction, full body inverse kinematics and the likes. But in the end, these are just gimmicks and without a game to back them up, they become old fast (see Bonelab).

On the other hand, if you minimize gimmicks and focus on gameplay, it turns out players won't mind the lack of "immersion" as much: both Pavlov and Compound has minimal VR interaction implementation compared to other titles, yet both are very successful (not to mention the huge success of HL2VR which barely have any).

However, as a dev it's easy to get lost in the details and add fun but ultimately meaningless additions which only increase the complexity of the game and take away resources from the actual game part.