The solution of “Just collect blood samples from all users” is the law. Gambling laws require that the company providing a gambling service verify the age of the user to prevent underage gambling. If Valve is providing a gambling service then by law they should have age verification. So far, legally, what Valve has been doing is not gambling and part of this lawsuit is proving that what Valve is doing is gambling, and if it is gambling then Valve is doing an inadequate job at verifying the ages of the users and thus is breaking the law.

I’d like the NYAG to equalize pressure on sports betting sites.

As far as I know online sports betting sites are already required to verify the age and identity of the user. Outside of changing the laws I don’t see what pressure you’re expecting.