Society doesn't allow it. Porn operators get into legal trouble for allowing access to kids. Movie theatre operations get into serious trouble for allowing kids in to see movies they are not supposed to.

Your argument of "let the kids have everything and make the parents perform the restrictions" is so stupid it's literally never been tried.

> I’m actually not that invested in ideology or whatever.

I find that hard to believe, considering

a) you reacting to an "age-appropriate material" argument being no different from a reaction to "no sex ed" argument.

and

b) what you are proposing, in general, has literally never been successful before. ALL societies places restrictions on kids whether their parents like it or not. Some are more restrictive and some are less, but they all have restrictions exclusively for children.

Because it is impossible on a practical level to do so with a public school system and a constant low-intensity campaign to infiltrate schools with pornographic garbage.

How many hundreds of hours of spare time per year would a parent need to expend to discover the 3 books they don't personally want their child to be exposed to?

For that matter, would their wishes even be respected? Or would we have school employees letting them read it on the sly, as some bizarrely-conceived revolution against the oppressors? There's more than enough evidence of schools taking the opportunity to hide disturbing developments about children from their own parents.