I remember when I was little I used to ask my Mum for her email address so I could play games, I think one was ‘farmville’, and hotwheels.com.
Hehe, gosh that takes me back.
But as mentioned, most players of roblox are very young, most won’t know what an email address is, and some may not even know their birth date - so this can be a deterant, as EchoRepear said.
Age could be added later, like email, and ROBLOX could simply presume the new account is under 13.