I mean, in this case because it's a standalone device, so... for the same reason you log in to your PlayStation. Also, you already had to log in to it when this was a Oculus thing, the "I don't want a Facebook login" complaint only became a talking point after they transitioned from the Oculus login over to the Facebook login, so the intellectual honesty in moving the goalposts based on this argument seems dubious.
In any case, I could see you getting uppity about logging in to use it wired. Maybe. There are a ton of hardware settings and configuration that are handled within the Quest's software directly, so I bet that would be way less trivial to deploy than people imagine. There is certainly no way I can envision where this thing would be usable wirelessly without a software login. You need to run an app to link to your PC, be it the Oculus or the Steam Link app. For security reasons alone you don't want a logless device that streams what's on your desktop monitor at will.
EDIT: Also, for the record, there are a bunch of monitor manufacturers that do ask for a login. Hi, ASUS Armoury Crate, you suck and have always sucked.