Lots of weird incorrect answers in the comments. MS 100% has changed CPU architectures and needs to emulate old games. The 360 was basically a PowerMac.
My guess - the Xbox One’s launch catalog was trash, and MS doubled down on emulation to build it out. Then they never stopped. They kept plugging away at it, and now they have a giant asset for GamePass.
MS got a head start because they were desperate for good games in the early days on the One.f
Weird thing is that Fillion is back as Cayde in the final shape promos.