The reason is a technical one. At a strip club, none of your information is being transmitted; it’s just the bouncer making sure you’re of age by looking at your ID.
Per the EFF:
Age verification systems are surveillance systems. Mandatory age verification, and with it, mandatory identity verification, is the wrong approach to protecting young people online. It would force websites to require visitors to prove their age by submitting information such as government-issued identification. This scheme would lead us further towards an internet where our private data is collected and sold by default. The tens of millions of Americans who do not have government-issued identification may lose access to much of the internet. And anonymous access to the web could cease to exist.
https://www.eff.org/deeplinks/2023/03/age-verification-mandates-would-undermine-anonymity-online
I see what you’re asking, and I agree if we’re going to prevent physical access to strip clubs by minors, it makes logical sense to take steps to prevent minors from accessing prurient content online as well.
The question becomes the exact methodology used to achieve that. It’s the same basic premise of making encryption illegal: Are we willing to sacrifice our privacy in the name of “protecting the children”?
Come up with another way to restrict access that doesn’t further encroach on privacy. I don’t have the answer for what that is, and it may not need to involve the government, but allowing them to put bills like this in place sets dangerous precedent. Once we relinquish power to the government, it’s damn near impossible to get it back.