Facebook has connected many AAA gaming studios to its VR platform, so it is not accurate to suggest that Facebook hasn’t partnered with any. For example, Insomniac Games, Bethesda Game Studios and Ready at Dawn are all big game developers that have worked with Facebook to bring high-end AAA VR experiences to the Oculus platform. In many cases, these developers have collaborated on console versions of their titles, such as Insomniac’s “Stormland” and Bethesda’s “Doom VFR”, in addition to working on the PC version of their games.
That being said, there are still many AAA companies that have yet to partner with or release games on the Oculus platform. This is likely because of a few reasons. First, there is a relatively small pool of Oculus owners, which limits the potential userbase for any given VR game. A lack of userbase can be a major hurdle for some companies, as the cost of producing AAA quality VR games can be quite high. It is also possible that some AAA makers have not joined the Oculus platform due to the graphics capabilities of its hardware. The Oculus Rift and Oculus Quest are still far behind what consoles like the PlayStation 4 and Xbox One can offer, so ports of AAA titles may not look as good on the Oculus hardware as they would on current-generation gaming hardware.
Finally, some game companies may prefer to develop for VR-focused systems like the PlayStation VR or the Valve Index, which can offer a more immersive gaming experience than what is currently offered through the Oculus. It is also possible that many companies are waiting to join the VR gaming market until the technology and userbase of VR platforms have advanced further.
All in all, while Facebook has indeed partnered with many AAA game developers, there are still quite a few companies yet to join the Oculus platform. If the Oculus hardware becomes more powerful and the userbase increases, we could potentially see more ports of popular titles from game makers who may have been previously hesitant to develop VR games.