The blockers generally don’t work (I’ve tried). A big part of the problem is that it is in the interest of the providers to subvert them. The main benefit of age verification is changing that responsibility by making it the business of providers to ensure it works, not consumers.

You can make the same argument with alcohol: surely it’s sufficient that parents supervise their children not to drink. But it’s prove to be more effective to distribute that responsibility to the providers as well.