You don't have Windows 10 (pretty big and obvious reason)

It's literally free and available to 90% of the owners of PCs around the globe going by 2015 numbers. But sure.

You want to minimize the number of places that have your personal information

Yup this is a valid reason, I agree, though that can be applied to literally every website, every store, every platform, every email you have.

You don't trust Microsoft's policies, and have thus deliberately created a local account (which is the case for a lot of people, and is perfectly valid)

Once again yes, but the same can be said for Valve, who have notoriously horrible customer support, took a fucking decade to add refunds to their platform and the fear of legal repercussions, introduce untested features/content that may very well make the platform worse (e.g. paid mods, the revolver addition for CSGO) so the people supporting Steam but not the MS Store once again don't make much sense to me. Plus, using Windows already means you're giving most of your info to Microsoft and being subject to their policies, like it or not.

Any one of the issues with MS Store games, which people have alluded to in this thread.

Completely valid reason that can be used for all other platforms again. I mean Christ, how many shitty rip off games are on Steam because of their complete lack of curation? Huge YouTubers, including TB, have given light to such horrible devs releasing multiple games per week or sometimes even per day, but Steam has remained completely inactive on that front.

It could be that the MS Store itself doesn't work for them - I know the Xbox app was broken for me for quite a while a few months back, so it's entirely possible that the MS Store app could break for them.

This one is of course fool proof reasoning. But I am mostly sure that the vast majority of people posting these comments don't have this issue.