Why do you think shops ask for proof of age when you buy cigarettes? Not because they care about cancer or want to sell less, it's because they're required to by law. Of course, teenagers can still find workarounds. They can ask an older friend to buy it for them, just like they can use a VPN to access porn.
The difference is, regulation shifts accountability. It moves the responsibility from a greedy, insensitive business owner to the kids. And at least with the kids we can guide them, and help them spend their time and money where it actually matters.
Note: I know people who love guns or porn are probably going to downvote this, but someone has to say it.
The state doesn’t regulate these things to protect people, it does so to manage risk to itself. Porn, guns, gambling, tobacco, alcohol, etc., are tolerated so long as they are contained, taxable, and politically useful.
Regulating porn is this system likely trying to move the needle on declining birth rates. You can look to a host of pro-natalist efforts in China as the likely inspiration.
And without a doubt, overreach by governments will continue.