In football, you cannot be behind a certain point on the field at a certain point in the game in relation to the ball. You cannot cross that point on the field until a specific action happens.
It is illegal in football to be on the opponents side of the line of scrimmage prior to the snap of the ball.
Similarly, in soccer, you cannot be behind a certain point on the field until a specific action happens. You cannot be behind the 2nd to last defender prior to the kick of the ball.
Both are arbitrary rules as to when and where players may and may not be. But neither game is ruined because of the rule. It's the way the game is. Both can be enjoyed at high levels in their current form.