Whats the point of randomly going into a game without knowing if you're going to get matched vs silvers or lem.
If you're on the side against the better players then you waste 45 minutes not learning a great deal because you're just being headshotted every time you do something wrong, and if its you vs worse players then you don't have fun for 45 minutes because you completely stomp the opposite team.
MM puts you up against players closer to your skill so you can actively improve vs players your level which provides closer games which are more fun.