1) in order to cheat the system, you'd need to have a fake government ID
2) it records actual PEOPLE as scammers, not just IP addresses or names which can very easily be faked by anyone
3) when you turn 18, all your actions and illegal dealings stay with you FOR LIFE. You scam a few people out of hundreds of dollars when you're 16, it sticks till you're 18, and they wipe the slate clean.
4) I want a basis of character for anyone I deal with. For someone who has no trader rating, the basis of character is the fact you took it upon yourself to go out of your way to meet an age verifier and that someone has met you IN PERSON to verify that you are, in fact, a real person. And having dealt with the BS of getting AV'd makes that person much less willing to do anything to lose AV status.
Us 18+ members wanted a place where we'd ONLY be dealing with other people who are 18+
If you don't like it, nobody is forcing you to stay, so just go buy and sell your stuff on one of the MANY other sites available to you.
We like our system, it's staying.