Passwords often protect things like random niche forum boards from grief more than they protect the user's sensitive information in such cases. 3rd party auth is a great solution but a lot of people don't want to tie their "real" accounts to the low tier sites. MFA is of even greater help for low tier site's pains but if you can't get someone to use a decent password or link their identity how likely are you to set up 2FA for it? In the case of "free" services type signups they want you to onboard your information or link your identity and an account workflow is the easiest way to do that as it's a small percentage that will go through the trouble of burner or temp emails and fake info yet at least you have an easy way to rate limit such users from hijacking your "free" offerings.

Also you're not supposed to be memorizing anything for logins. At the very least you should be letting your browser use the randomly generated password and save it to the browser password store if you're not using a full blown password manager.