Actually, Larsen raised the same question.  He thought it was a positional mistake to trade off a center pawn so early--but then, he really liked center pawns.  Obviously, since so many play this, they think that getting freedom for their pieces and attacking chances is worth it.  However, it is no doubt one reason why so many times Black gets good endgames out of the Sicilian (if he survives to the endgame)--and why, as the proverb goes, short games in the Sicilian are won by White and long ones are won by Black.

After move 4, the position is unbalanced. This is what many players want. White is ahead in development, but Black is on the move. Each side has a different half open file. White's knight on d4 is well placed in the center and Black will often challenge it. Playing d4 unblocked a white bishop. Pawn moves are made to allow pieces to develop. The game is still balanced.