Why don't the air conditioning vent fans work when planes are on the ground?

They do don’t they? The only reason the cabin shouldn’t be air conditioned is if the plane isn’t powered or the plane is having problems with that particular system.

Because the cabin is pressurized when in the air, and those fans are tuned to specific air pressure. When at the gate with the hatch open the air is not circulated as this would cause the hatch to blow air in/out, not necessarily good as the air quality on Tarmac is poor b/c of the fumes from exhaust and jet fuel. So my guess is keep the air movement in/out of the cabin to a minimum while docked.

But it does get stale and hot sometimes doesn’t it?

@sndfreQ: Thanks. And yes, it gets incredibly stale and hot. It would be nice if there was a secondary system to get some air moving when the plane is on the ground.

With modern planes, don’t they just circulate air from the terminal now?