The lawmakers do understand that, you are allowed to use sessions and cookies for essential tasks like tracking login state, and no popup is necessary in this case.
I wouldn't be shocked if you are correct and many devs are indeed completely ignoring the legal requirements here, but that may be in part due to developers not understanding the legal requirements or believing incorrectly (as you seem to) that the requirements are not technically feasible.
A simple, GDPR compliant solution would be a tickbox for "remember me across visits" next to the login form. If ticked you get a persistent cookie, if unticked just a temporary session that goes away when the browser is closed. No popup is needed.