There's multiple issues with age verification.
Biggest of which is that we all know that data breaches are happening essentially all the time, the type of ID these sites would require e.g. passport including photo, is very juicy data for ID fraud merchants so privacy conscious people may not want to risk giving that data to a third party*
As the article states, ID is also fairly useless for porn, as there will always be sites in different countries that people can use so local laws irrelevant, torrents of erotic content etc. It is not a vital service that people need access to so all ID demand will do is stop people using providers that want ID.
* In my dealings with banks, I have made the (often irritatingly long) trip to a branch with my ID, which got photocopied to be checked, but not stored electronically, rather than submit it online - but I'm a bit extreme on security issues (having had attempted ID fraud performed after address change house when bank were tardy in updating my address details & sent out a statement to old address & new tenant or a mate of theirs then used that to take out a loan in my name & it was a real PITA to sort it all out, even though things like signature used by fraudsters was totally unlike my signature, bank refused to tell me what other ID was used, if any!, (as any ID beyond the statement would have needed to be forged) - only figured out the statement must have been used when I contacted bank to ask why I had not received statement at my new address )