It's not about buyers, but publishers. If a publisher gets 95% of revenue by being exclusively marketed by MS, versus 70% on Steam, they might team up with MS instead. Surely if MS dominates the space after some time, they increase fee, but nobody thinks more than a quarter-two ahead in gaming industry.
I think in most cases you're correct, but store exclusives are somewhat common right now, and users tend to hate it (google "metro exodus controversy" and you'll see what I mean - and that wasn't even exclusive to one store, you just got a discount for buying it from Epic).
That said, I don't have access to any data about sales - it's possible that just as many people buy it regardless of what store it's sold on. But I would suspect that is unlikely given how easy it is to pirate something and gamers' tendencies to believe they're owed a game.
