What happens if the government can now perfectly enforce that people under 18 can't do X or Y?
It feels healthier for the enforcement apparatus to have a budget, in terms of material personnel or time, that requires some degree of priority-setting. That priority-setting is by its nature a politically responsive process. And it’s compatible with the kind of situation that allows Really Quite Good enforcement, but not of absolutely everything absolutely all the time.
Otherwise ossification feels like exactly the word, as you said, stavros: if it costs nothing for the system to enforce stuff that was important in the hazy past but is no longer relevant, nobody wants to be the one blamed for formally easing restrictions just in case something new bad happens; 20 years later you’re still taking off your shoes at the airport. (I know, I know, they finally quit that. Still took decades. And the part that was cost-free—imaging your genitalia—continues unabated.)