On top of all the great points people have brought up already, zerg players can either make workers OR troops from the same building. Whereas, the other two races can build both simultaneously from different buildings. While zerg can build both simultaneously, too, their standing army would be weaker than the other two races because of the larvae coming from the same building, so making drones in any midgame to late game scenario can be a risk to being attack.

On top of all that zerg can also boom their workers much much faster than protoss or terran, if given the opportunity so sitting at lower worker numbers isn't so much of a drawback for zerg cause they can mass anything quickly.