Even if they were somehow made "smart" enough to navigate all the obstacles we face on a daily basis, the problem of battery power is insurmountable. The most radically advanced battery technology we have today is still many generations behind what would be necessary to make robots of any kind a viable delivery system. Look at the size and weight of the battery pack on a forklift that moves pallets around in a flat warehouse and then try to imagine that amount of power being available to robots or a drone that might have to travel many miles to deliver a single package. The technology for such a battery simply isnt there.