> Sure, but the vast majority of websites where I see these cookie prompts aren't even websites I'm logging in to in the first place... often they are websites I am not sure you can even log into at all.

Cookies for managing login's are explicitly allowed by the GDPR. If you get a cookie choice prompt when visiting a website, it is an indication that the site is placing "advertising/tracking" cookies, for which the GDPR does require consent.