At those frequencies you're going to be losing a lot of power in the switching losses of the transistors. Power MOSFETS have very low leakage when fully on or o

At those frequencies you're going to be losing a lot of power in the switching losses of the transistors. Power MOSFETS have very low leakage when fully on or off, the largest loss tends to be during the transition when voltage and current are non zero. Because the speed of that transition is limited, the higher the frequency the more percentage of total time is spent in those transition phases and the higher the losses.