Why would they bother?
It'll just cost more time, money, and resources to port something that doesn't need to be ported, as it will still reach the exact same audience without it.
Why do EA and Ubisoft bother creating Steam and Epic versions of their games? They're all on the same PC platform. Because certain users prefer certain ecosystems, each with their own advantages and disadvantages.
Bold of you to presume that the hard-core userbase that stuck with Xbox, with large Xbox libraries, and only prefers Xbox Console and controller gaming will simply migrate to EA, Ubisoft, Steam, Epic versions of games.
Xbox Store will be the default front facing store. If devs want to sell on that store, they will need a Xbox ecosystem version. If devs want a Gamepass deal at launch or 24 months later, they will need an Xbox ecosystem version. If devs want to have their game on xCloud, they will need an Xbox ecosystem version.
If devs want free MS marketing, they will need Xbox ecosystem version. In case of Rockstar, they are only doing and maintaining Console versions at launch. In case of EA, if EA still wants EA Play to be included within Gamepass Ultimate in the future, their games will need to have Xbox ecosystem versions. They have been included for 5 years now, it's a very lucrative deal for EA.
Publishers also prefer secure, closed platforms that can prevent piracy or cheating.
Just because MS allows third party PC stores, doesn't mean MS would be promoting or selling other Ecosystem games. They made a commitment to being an open platform, not an obligation to improve games and versions for other platforms. Xbox ecosystem versions will have certain advantages the users want and the publishers need.