I think the key for Facebook (and Amazon) are your 2nd point. People login once, and likely never again.

I got my mom, who is in her 70s, a new TV and wanted to sign her into Prime Video. I asked for her Amazon account and she had no idea. I think she said something like, “I don’t have an Amazon account, I never have to login. I don’t even know what it would be.” She has been a Prime member for a decade and hasn’t had to login for so long that she forgot she had an account. It took both me and my sister telling her she must have an account, and listing the reasons why she must have an account, to jog her memory or simply convince her that this was a reality.

This creates a different problem. People forget passwords, lose account info, and when they do need it the recovery is that much harder. Apple Keychain has saved the day with my mom several times.

I was a 1Password user for about 18 years (recently migrated away), and it would ask for your master password every 2 weeks and this is why. If you only have one password, you better remember it. If people only have to login once, they’ll forget. There were a couple times over the years when I drew a blank on what it was and got kind of worried. I also always worried about what would happen if I had some kind of head injury, as I never wanted to actually write that password down anywhere.