You can claim the MS Store is an outlier because "nobody uses it" (maybe? got numbers? it's obviously not #1, though) but other stores offer lower cuts as well. Epic made a big deal out of their lower cut (~12%) compared to the other stores and developers seem to be paying attention.
Mostly because they're throwing buckets of money at the developers for exclusive deals, not because it's a better product offering in any way, or has a better audience.
The Epic store doesn't need to make money, they have Fortnite for that. A barebones store missing features that even the worst online marketplaces have, is just their attempt to become the new "monopoly" if they can get enough of steam's customer base. Once the platform matures and they stop throwing money at devs and giving away free games, we'll see what changes they make, if any.
If the software I want is on it then I use it, while Mac App Store is an inferior experience to a downloaded app + update framework like Sparkle I've found Microsoft Store far superior to software updaters and installers that usually ship on Windows and it works much faster and smoother than the Mac App Store which is very janky.
I don't think negative economy of scale applies here.