It's good to hear that keyboardists are in such high demand.
The result of such imbalance can (in my opinion) be attributed to several factors:
1) Keyboardists do not play a fatal role in bands.
You already mentioned that keyboardists are there to make the music even better. So without one, you can still make good music. Just need drummer, bassist and guitarist.
2) Keyboards are heavy.
You can sling a guitar around your back. You certainly will find a drum set at gig locations. But a keyboardist has to bring his own 20kg bulk. And one may not be enough.
3) Keyboardists can make their own music.
Keyboardists do not need bands to make merry tunes. So we are not deprived of seeking bands. On the other hand, drummers can't drum without accompanists - bassists likewise. Guitarists are ok, but hey, you don't learn electric guitar to play alone, do ya?
4) Good keyboard players are well-trained in classical piano. Pianists don't learn the instrument overnight.
More adults pick up drums and guitars than keyboards/piano, because piano is more difficult to learn. So to find a freshie keyboard player is more rare than a freshie guitarist.
5) Keyboard skills are not simplified
Technically-speaking, the keyboardist that controls the synthesizer with his 10 fingers can produce all kinds of tones you can ever imagine (one reason why keyboardists are so widely sought-after to generate the sound that no other band players can achieve). Such power, yet the perceived role is myopic. People think of keyboard players as pianists, therefore there are not many courses specialise in training the actual management of the keyboard, which is not totally about pianistic skills. Afterall, the keyboard keys are so light, you don't need years of training to achieve versatility. If someone can produce a simple curicullum to pick up keyboard skills in 3 months, I believe the supply will soar.
6) Good keyboardists do not have time for gigs (isn't it?)
They are probably too busy working with session musicians in recording backing tracks for the next pop idol.
I may be exaggerating in some scenarios, but in a nutshell, it's probably more difficult to BE a keyboardist, let alone finding one.
Cheerios!