Nice, that explains the crossover.
I'm sure someone more educated could provide a better answer, and possibly even an alternative to what I'm saying (some exclusives may use proprietary engines, but even that wouldn't change much, the HMD is just a display).
I got into it with the Croteam developers a year back over their overpriced "ports". They released Serious Sam 1-3 with minor VR functionality (room scale / motion controls). They charged way too much, pretended it was a lot of work (it's not) and started lashing out at the community.
Porting a game to VR is as easy as porting a game to 4k, or Eyefinity / Nvidia Surround. As in there won't be so much of a port at all, rather a discrepancy in the interface or game play that was never intended to be viewed in that capacity. When you add in motion controls and room scale? Sure it gets more complicated, but honestly, not by much.
https://assetstore.unity.com/packages/tools/vrtk-virtual-reality-toolkit-vr-toolkit-64131
Not to take away from any of the developers porting stuff, but it's practically point and click. You can make any game a VR game with "no" coding experience (see; vorpx), and even getting the mouse / weapons slotted to the motion controls, combined with room scale is as easy as setting properties to an asset.
EDIT:
I'm not sure how many people realize this, but you can play almost anything in VR today.
Kingdom Come: Deliverance, Metroid Prime 1-3, Mario Kart, Witcher 3, World of Warcraft, etc.
GTA5 has multiple VR mods, some with motion controls (absolutely amazing, like shockingly cool stuff), some with controller support.
Here is a list of nearly perfectly supported non-VR games, in VR.
https://www.vorpx.com/supported-games/
And this list is highly incomplete. I was playing Quake 2 the other day in VR with full motion controls, I beat Half Life (the original) in VR with full motion controls as well.