The ISP verifies your age when you buy internet access. The clerk verifies your age when you buy wine.

The clerk doesn't follow you home and make sure you don't give your kid wine. And, symmetrically, the ISP doesn't make sure you don't allow your kids access to porn. Once the transaction is done, it's in the parent's hands.

The difference is that parent's aren't very keen to give their kids alcohol... usually. Although it is explicitly legal in a lot of places. But for the internet, they are very keen so it's normalized and nobody cares.