Can you explain why is there zero voltage at the source of a differiential MOSFET amplifier?
As stated in the caption this is a small-signal analysis.

The inputs of the amplifier are driven differentially, when one is driven with a positive voltage the other is driven with a negative voltage of the same magnitude but opposite sign.

For this reason when for example Q1 is driven with vid/2 we have a current gm*vid/2 flowing downwards through this transistor. At the same time Q2 is driven with -vid/2 and the same current is flowing upwards through Q2.

So even if there was conductance from the "virtual ground" node to another node there would be no current to cause a voltage drop. Hence the small-signal voltage is zero.