If you look at the schematic diagrams of the audio output systems on most keyboards, you will quickly discover that when you plug into the mono jack, there is an internal switch that combines the left and right channels - thus MONO! No different than plugging the right and left outputs into to 2 channels on a mono amp, mixer, etc..., it's still mono.

The good news is that no one in the audience gives a damned.

Sure glad I drink,

Gary