For the same reason a bars don't just ask people to sign a document saying they're 21 in lieu of an ID. The laws are written in a way where the responsibility

For the same reason a bars don't just ask people to sign a document saying they're 21 in lieu of an ID. The laws are written in a way where the responsibility for enforcement falls on the operator of the business. In both cases, the business doesn't actually have to verify anything if they don't want to, but if it's found that they're allowing violations to happen, they will be held legally responsible.