Kaz, Apart from the extra cost of producing a larger gearbox ..
and the fact that a bike can be manoeuvred to very near where you need to park and that a bike can simply be pushed into a better position ........
Have you seen some people try to ride a bike FORWARDS let alone backwards.
Add in the factor that some people just cannot reverse a vehicle even when it has four wheels on the ground, so is therefore not going to fall over, imagine how many bike will topple over.
ME !! cynical .. .. Never ...