I'm just curious.

Games that I've been looking forward to finding online again for awhile now are the Age of Empires, Age of Mythology, and the the Black & White series.
I'm kind of surprised they haven't, some Fable games have gone digital, and the Empire games may not have been blockbusters, but I think they were pretty well received. They seem like the kind of things that would go digital, if not here then somewhere, possibly even with their own developers, but so far nothing. :S
Is part of it because Ensemble Studios was disbanded?

More like Microsoft just doesn't give a damn about PC games anymore. See "Games For Windows" on a site called Xbox. Yeah.

I'm pretty sure AOE3 was like a penny on the MS digital store a year or two ago (seriously, it was a penny, might have been AOE2, I don't recall anymore). They're not impossible to get digitally, but honestly the discs are still readily available in those cardboard jewel case things, you can get AoE2 plus expac for 10 USD at Walmart.

MS owned Ensemble studios, they have the right to do anything they want with all those games. That's why Gas Powered Games is doing Age of Empires Online (which gets a major overhaul in about a week).

Black and White was trash to me, so I haven't tracked whether you can get that POS still or not. I probably literally jumped up and down on those games a decade ago. I can't help you there.

But for the MS titles, keep checking the MS store (attached to XBox Live store, yes). Newer AoE games are available on a lot of digital platforms (AoE3 is on Steam, for example).