I think the answer is that none of the big companies want to be completely left behind when it comes to the big three technologies (AR / VR / AI). Even though they don't have any big VR or AR hardware this is a way to say they're investing in the space and carving out a niche. They see these engines as a way to sell AWS offerings. One area being servers for games, another being bots and AI and a third being hosting for immersive media. Some immersive technologies will have huge requirements in terms of bandwidth (for instance, offering high def 360 video with lightfields or depth information). They haven't come to fruition yet, but in a decade, who knows?