Why does the Darlington transistor get so hot?

The chip inside the TIP120 is supposed to be as cool as you can get it. It passes most but not all of its heat to the metal tab which heats less and your insulator prevents some heat from going to the heatsink. Then of course the chip is hotter than the heatsink.
The amounts of heat losses are listed in the datasheet of the TIP120 the same as its schematic that shows its base-emitter resistors and amount of leakage current when it is cool. Its leakage current is Much Higher when its chip is hot.