But your mobile device is also only one device you interact with in a day. Very few people have the same OS across all their devices, even if you allow for the "close cousins" scenarios like people that only use iOS and macOS. Everyone uses the web everywhere, though, it's the one thing guaranteed to tie all of a user's devices/accounts/actions/"digital life" together today.

I don't have numbers to back it up, I have a lot of anecdotal evidence I've seen, but it does look like there seem to be at least as many people that don't care what widgets an app uses, so long as the app works, as there are people such as yourself that even notice native UI widgets and care about them (deeply and vocally, as often the case may be).