Because we live in a society and things that impact society impact you, as a member of society. It’s the same reason why we have age restrictions for alcohol, porn stores, and cigarettes. It’s also why we have laws about seatbelts, labour, and certain non-toxic but excessively unhealthy ingredients. Even if you take giving a shit about others out of the equation, the self-interested view knows that what happens to others’ kids now can and do become your individual problem down the road.

Enforcing age restriction is not a ban. I think as an adult it’s entirely your perogative to watch your dwarf porn. However your framing of “making everything kid friendly” is a bit misleading and disingenuous. It’s quite the same as complaining everyone is always trying to make things kid friendly because they check your ID at the liquor store. Would it be more convenient if you could just grab and go? Sure. But the social harm without it clearly outweighs the inconvenience.

The fickle problem with the internet porn is still privacy and data security, but that’s a separate issue.