Why A Class D amplifier does not operate its devices in the linear region at all; the transistors are driven fully on or fully off like switches, usually from a pulse-width-modulated drive signal, and a low-pass output filter recovers the desired waveform. Because a switch that is off passes no current and a switch that is on drops almost no voltage, device dissipation is very low and efficiency can exceed 90 percent. The tradeoff is that the raw output is rich in harmonics, so output filtering is essential.
Watch out The description of an amplifier biased for low distortion points to linear classes such as Class A or AB, which are the opposite design philosophy: good linearity paid for with low efficiency. 'Drift-mode FETs' is not a real amplifier class.
D is for Digital-like switching: devices are either fully on or fully off, so efficiency is high and a filter cleans up the output.