Why do my parents completely bar me from all and any games (literally)?

I was thinking about getting Portal 2 and after scrutinizing it for a while I think its an awesome game (the main reason is I want to co-op with my friends who also play it), especially since that it also has a E10+ rating and its PG - very little violence. But how do I persuade my parents to help me get it? Whenever I bring up any related topic about gaming, they get irritated saying I have no time since I must study (which is true, I study alot). But I don't think this means I cannot get one computer game that is safe for any teen to play, is there anyway I could persuade my parents to get me it?