Ummm... because the driving experience is a compromise between a number of competing factors?? Let me design a maximally
efficient car for you and I guarantee you wouldn't want to drive it
I'll take this opportunity to once more mount my hobby horse and say that the answer is to
build regeneration into "braking" (i.e.,
decelerating) so that the driver doesn't have to manually decompose deceleration into non-acceleration (i.e., foot
off the "gas" pedal)
and braking (i.e., foot
on the brake pedal).