If online casinos were really checking and trying to ensure that a gambler behind the screen is an adult, that business would not be so popular and profitable today. As long as people keep bringing and losing money, the casino does not care about the age of a gambler. They only start to care a little when gambler tries to withdraw large amount, his gambling session is short, he isnt much active on the platform and his playstyle is suspicious. Why online casinos does not bother about age check also, is because there is no strict supervisory authority and casinos licenses does not cost a lot.
Agreed, the measures to determine the age of the players seem to be too sketchy, more precisely, this rule is only for dealing with government regulations because they can justify that they tried to prevent it but the underage players still know how to fake documents and pass the check, in the end the casino denies all responsibility. Honestly, there are too many strict ways to check age but that is the way that reduces profits too quickly, casinos cannot negotiate with the government on these issues and instead of fighting back, everything will seem more legitimate but come with loopholes