I'm speculating here, but it wasn't until EGS launched that Microsoft felt the need to start selling games on Steam. MS doesn't sell on EGS, either, except for Outer World's but that's just because that deal was in place before MS bought the developer and it's published by another company. Also, there are a couple of games like Outer World's and Metro Exodus that are considered EGS exclusives but are also available on the Windows Store.
I sort of think MS is more threatened by newcomers into the market than they are of supporting Steam at this point.
MS were selling titles on Steam long before EGS came about. For example, the Age of Empires series has been on there for years and years now. Halo: Spartan Assault has also been on there for over 5 years now. State of Decay for about 7 years now. Lots of other examples.
For their console exclusives, sometimes they waited a few months to release on Steam, but that was due to trying to push the Microsoft Store. When that failed to take off like they wanted they started releasing on Steam as well.
I'm guessing due to how well those titles have sold on Steam compared to the Microsoft Store combined with their push to win consumer goodwill, they are now pushing titles to Steam day and date with the MS Store.
As far as consumer goodwill goes, it's one of the moves they've done that's helped turn around their image with consumers.
And yeah, the Outer World deal was inked before MS acquired Obsidian, otherwise it would have appeared on Steam at launch.
Ideally, I'd love to see their titles also appear on GoG (and other storefronts just so consumers have more choice), but since GoG won't allow DRM on their store, that isn't going to happen anytime soon.
Regards,
SB