To save time, since little engagement would happen until pawns reached the center, but just tedious moves by both players to get them there one square at a time.
You don't have to do it. You can do it. And to compensate for it they came up with the rule called en passant. You can Google it. En passant allows you to capture a pawn that advanced 2 spaces as though it had advanced 1.
When is a pawn allowed to advance two spaces in the middle of a game?