Microsoft has it’s own appstore, they have internal expertise, but different business unit (or division or whatever the windows appstore falls under). The MSFS marketplace does integrate into the MS game/XBox ecosystem as well though on some level I guess, so there is a level of coordination going on there that involves some organisational friction too, no doubt. Don’t know though, pure speculation.
However, I suspect the MSFS marketplace platform rate is tied to that, as it mirrors that it seems and MS is the one out of step with Google/Apple now on platform rates (ie: the ~ 30% store dev tax).
In any event, there are many appstores out there with many simple to complex products that have to be approved in a pipeline in accordance with dev agreements and tos, and deployed to multiple OS’s and hardware devices. This is not a unique situation on that level. It should even be possible to automate testing of some assets to see if they conform to certain design paramters.