Ultimately, they seem to have realised that they can't stop adult content from being shared, so the easiest way to get there was to mark anything even vaguely possible of being adult, and require age verification -- which comes with a lot of political cover vs. just deleting it.
Of course, if you stoke up the right people, you end up with lots of support from the puritanical brigades, and label all naysayers as putting children in harm's way.
If you voluntarily sensor content, you might be in danger of being held responsible for various things since you control what people see. Phone companies in the US are "common carriers" which means they just connect people, but are not responsible for what people do over the phone (plotting crime or whatever). Social Media is still trying to have it both ways - censor some stuff but not be responsible for anything. IMHO that will eventually fail.
I guess they figure if they keep trying they'll eventually get it passed - which is probably true.