Sometimes it's not the fact that you have to sign up but the way registration is done.

"people will just pick their normal username and mash a laughably simple password into the appropriate box"

It's not as easy as it sounds.

1) My username might be taken 2) Often, I have to go to my email and activate the account. 3) Entering and remembering a password is not as simple as it sounds. New York Times is a good example on how not to do it: http://twitter.com/vpdn/status/1620213111 4) Most of the time I don't see why I have to sign up.