Power rating is primarily based on the ability of a system to get rid of heat. There are basically thrre methods which are conduction, convection, and radiation. Mounting a semiconductor to a heatsink is an example of conduction. A cooling fan is an example of convection. Black anodizing a heat sink is an example of radiation.
If you look at the equation of a diode you notice that the current is an exponential function of the forward voltage. Past the threshold of about 0.7V, small increases in the forward voltage produce large increases in forward current. At some value of forward current the device will proceed to get warm, hot, and vaporize. It is the vaporization that effectively limits the current without a resistor. It should not be a surprise that this thermodynamic process is not reversible. So far as I know there have been no recorded cases of molecules spontaneously forming a diode.