[mrtksn]

In my understanding, normally you have a tiny chip that is inside a larger package and then you can attach that packaged chip to something that takes away the heat. So the ratio of cooling element(package + heat remover) to heating element(the IC) is pretty large. Also, the speed of removing heat from the silicon IC is limited to the heat conductivity of the silicon itself and the materials used to make the package. The silicon itself is not where the heating happens but on the "etched/printed" features on top of the silicon.

Therefore, the amount of heat by heating elements(the tiny wires over the wafer) grow faster than the heat dissipation capacity, which rises the temperature until the unit breaks.

Notice that when the heating elements are close together you lose the horizontal heat gradient advantage since that grows by the perimeter when the heat generating elements grow by the area.

Which means you have to get creative, add moving parts or use more exotic materials, which makes the thing significantly more expensive and less reliable as more things to break are added.