They don't, the agent at the desk is just making sure that the name on the ticket matches the name on the ID you provide. There's been countless articles about

They don't, the agent at the desk is just making sure that the name on the ticket matches the name on the ID you provide. There's been countless articles about how insecure this is. You buy a ticket under a false name and print out a second one that has your real name on it. The airline's no-fly list checks the fake name on the real ticket you bought but the TSA agent checks the real name on the fake ticket matches the ID that's handed to him.