Why doesn't Earth's crust layer continually melt into the mantle layer?

Heat is being lost from the mantle through the crust.


The layer we see represents a complex equilibrium.


Lower melt point crust is lost to the mantle.


Higher melt mantle is deposited at the base of the crust.Why doesn't Earth's crust layer continually melt into the mantle layer?
Think of the temperature difference between the mantle and the crust. Now think of the pressure differences, if magma is to surface, it really has no reason to be extremely hot so it cools depending on temperature differences. The main reason: pressure.Why doesn't Earth's crust layer continually melt into the mantle layer?
It does at the subduction zone and also locally at the hot spots.


thnks
at one point, it did. but our planet is slowly cooling.