why torque motor doesn't overheat if stalled for very long?
A torque motor is designed to be run at stall. Maintaining torque at a fixed shaft position is exactly how you expect it to be run. 

The motor is designed so that its thermal characteristics allow this operation. The windings are relatively thick, so the Joule heating is minimized, and the case construction allows good transfer of internal heat to the surrounding structure. The windings are normally located far from the shaft axis, so that the product of magnetic force and radius of action (torque) is maximized. This in turn means that the windings are well-coupled thermally to the case, and there is relatively little heat dissipated deep within the motor.