Someone on the Crate & Crowbar podcast (I think Tom Francis, of Gunpoint fame) made the great point that it's much closer to footbal than FIFA is. FIFA resembles football more closely, but Rocket League actually plays like football, from a player interaction point of view. In FIFA, you can't control a player's legs, feet, balance, the precise way they hit the ball, etc. We just can't simulate that very well, much less control it. But we can certainly control the trajectory and orientation in space of a rectangular cuboid hitbox.
In other words, you can actually dribble and kick a ball by controlling a virtual RC car, while in a sports game like FIFA, that's only an animation.