why are road shoes so f*in slippery?

I think mine are Shimano R078 or something similar. they look good, theyre comfortable and pedal nicely, however, if I miss the cleat engagement they just shoot off with there being no grip underneath.
Same goes for walking, if there are any stairs or pavements etc that need to be walked on, they offer next to no grip in the heel.
Why is this, what would be wrong with having some tread a bit further back on the shoe?
Are there any better designed shoes than this, most of the ones Ive seen in shops are the same.

I know I could change pedals and use my mtb shoes on the road bike too but that kind of defeats the object of having lightweight shoes and pedals really.