Also the lower-leg muscles, the calves and shin muscles, only exert force by pivoting the foot at the ankles. The further back you go on the foot, the less power is exerted by those muscles. If you pedal with your heels, zero force from the calves would go into the pedal-stroke.

Also think about when you have to exert a lot of force with your legs, like pushing a car or wrestling someone, where is your weight resting and pushing off from?

Simple test on the bike, put the pedal at the very bottom of the stroke and lean up against a wall to hold yourself still. Put the ball of your foot on the pedal and push. Notice your calf tightening up and pushing down on the pedal. Notice that the force pushes up on your body? During the dynamic motions of a pedal storke, the weight of yoru body counteracts the push on the pedal and drives it down and around.

Now put your heel on the pedal at the very bottom of the stroke. Tighten and push with your calves, what happens? All that force that used to push your body upwards now just goes into thin air by flapping your toes up and down, not pushing on the pedals at all.