I live in USA I just turned 21 a few months ago. When I go to buy beer or cigarettes half the time the cashier will just ask for date of birth instead of asking me for my ID. Why would they not just take the few extra seconds to look at my ID even though it puts them at risk?

I live in USA I just turned 21 a few months ago. When I go to buy beer or cigarettes half the time the cashier will just ask for date of birth instead of asking me for my ID. Why would they not just take the few extra seconds to look at my ID even though it puts them at risk?