Look at it this way: First the pedal moves, then the electronic throttle has to move, etc. You just can't suddenly shove that much more gas/air mixture into an engine in a milisecond. It might take a milisecond and a half. The engine has to get more air and the fuel pump has to pump more gas and the injectors have to cycle, and that's not all just instantaneous. It's actually a sequence of events that have to happen.