Do you think forbidding kids from having cigarettes or hard drugs is tyrannical?
So, to try and explain why these restrictions are more easy to enforce and ALSO less of a risk to society at large.
Firstly, let's say we restrict porn via an age gap similar to how we do with tobacco and alcohol. How will websites then do an age verification for their users? They will require some form of unique identifier only an adult has access to. This will either take the form of a Driver's License or Credit (not debit) Card.
Now, when dealing with over the counter purchases like alcohol and tobacco, this check is easy. The cashier looks at the card, compared the picture to the person, and checks the date, then hands back the card and completes the transaction. Now, fake IDs are a thing, but they are of limited distribution and the security features of ID cards has been getting better and less copyable.
However, there's a problem with doing this online. Firstly, you can't just scan the card in, not everyone has a scanner and subsequent manual verification for a site like Pornhub which sees hundreds of thousands of unique hits a day, isn't really practical. So you automate the process by using the unique number assigned to the Driver's License which the user has to input manually along with State and possibly some other information depending on what's needed to verify the ID. This is where the entire process diverges sharply from the in person process.
You see, when you buy alcohol or tobacco and show the ID, the cashier, unless they have a photographic memory, likely quickly forgets the data on the card. They don't remember you full name, address, the unique ID number, or anything else. They also won't necessarily make a record OF the card in any way, shape, or form. Once the transaction is complete, it's over.
However, in order to compare the ID data to the State database of drivers licenses, Pornhub would have to
retain that data, and this introduces a new risk for both pornhub and users, as if pornhub is hacked and that data stolen, that's a lot of personally identifiable information (PII) that is now out there, ripe for identify thieves to use.
Now, I'm not saying this makes the entire idea of such a system impractical, far from it, many websites contain and secure PII quite well (Amazon has home addresses and credit card numbers stored, etc.); however, my only point is that while it may seem superficially like a checkout card check and requiring ID before logging into a porn site is the same, from a risk and back end side of things, they're two VERY different beasts with considerably different risks.