Because right now the match engine doesn't play for players at that age. It's a sad fact but every skill formidable and below all act as disastrous and it is basically a complete toss up. I've found there to of course still be some randomness with better players but they seem to at least SOMEWHAT fit a certain position.
Right now you could literally switch the defenders and attackers on the U21 team and get no different of a result.
I am not qualified to be U21 coach. I managed to be demoted while Slambot stayed. I am also unfit because I think that the US Congress is an admirable political body with the country's best interests at heart.