It's worse than "forgetting." Having seen older folks just set up new accounts for a move, they make zero attempt to even try to keep them! Oh, the phone company needs a login/pass? Just type in anything, don't write it down. If something goes wrong, they're going to call in anyway, not use the website.
One-time logins actually sound useful for things like setting up utilities for a house. Sign up, log in, do whatever you need to do, log out and the account is immediately locked. Nobody expects you to log back in anytime soon, anyway.
If you ever need to interact with the service again, you initiate account recovery using a combination of your contact info and some codes printed on your monthly bill.