Your comparison won't work here. One involves someone physically walking into a place to purchase a physical good. The check is as simple as to look at a valid ID and comfirm that the person in front of you matches. Yes, fake IDs are a thing, but they're the exception and pretty hard to be convincing with.
Online this is significantly more difficult to handle without some heavy surveillance. How many times have you confirmed you read something when you didn't, or that you where allowed to access something when you where not? A simple checkbox isn't going to stop anyone from lying. For things like tobacco purchased over the internet there's at least the protection that short of fraud if someone had a credit card it can be validated that they're of age.
The law is performative at best. Even if you got every major US site to comply, new ones out of the reach of TX are all over the globe, not to mention avenues like BitTorrent.