Question

why the earth is not in thermal Equilibrium with the sun ?

Answer 1

Who says it's not?

Answer 2

Because the sun is transforming energy into heat, and therefore it gets a lot hotter than the earth. Also, it provides the energy that we need here to survive and keep the temperature into a livable range.
But as all things, it takes some time to get to equilibrium, and this process will end when the sun dies, and stop creating heat.
When that happens, all planets and the sun will begin to cool down until they are at equilibrium.