It's the phones. We live in a world where everyone with a phone makes an even more useless account just to use the phone. Ironically, also a car if they own a certain model xD, as someone pointed out above.
Don't get me wrong, you *are* correct. But the masses don't care and just move along, and you and me and the OSs follow suit.
Unlike the phones, technically it isn't neccessary to ever use the Microsoft account. They'd probably get into legal hot water if they did that, so instead they make it seem like you have to. All the "features" and offers are entirely skipable. I'll be switched if i ever logged in.
Deviously, i think they force you to sign in if you want to change your username. But that's not a part of OS or Windows i care about so, whatever.