This is my personal take: People are sometimes tricked by natural human instinct into performing actions that give benefit to a simple human need ("it's just easier for me") at the detriment of higher-level outcomes (in this case, password security).

It's simply easier for me, as a human, to remember that my password for all websites is Hunter2, rather than spend the extra time, create a password manager account, store passwords, utilize best password management practices, etc. Not saying this is what I do, but for many people, this is how they remember their password(s).

Maybe I should have changed the "tricked into" to "trick themselves", but I'm just a human and this was easier for me.