Hi all,
this is the problem with today's gaming (and not).
you think you buy the game, but you actually not. You buy the right to use it (sad but true), nobody own a thing, if you stop using the platform (steam, Xbox, PlayStation.....) you bought the game on, you can't use that game that you "purchase" anywhere else. Steam is the great example you can family share your library but you can't use it at the same time (I bought game X and Y, if my sharer play X and I decided to play X or Y, as owner I have priority and the sharer get disconnected). Microsoft that force you to log in at the startup, and force you to use the same account to make purchase (you can't log in with account X and have anything related (MS store, Xbox) using different account or it will ask you to log in with the right account or use the credential of the login at startup.
I don't know if it make sense explained that way, but that is the "complex" explanation. In few words only one person have the right to use content of an account.
Steam allow share because the second option could be log in a second machine and use the same credential as the first (without parental control availability), but then the first would be interrupted cause a second machine tried to use the same content.