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Serious answer time: casual riders, mountain bikers, and commuters are more likely to wave because they're sitting more upright and have a wider field of vision. But a rider going fast on a road bike has a reduced field of vision because they're leaned forward and more intently focused on what's immediately ahead or alongside their intended path. So plenty of times when I'm on a road bike I'll spot another rider on the other side of the road, but for such a short moment that I simply don't have them in vision long enough to go complete the 'spot, register mutual recognition, form intention to wave, execute wave' process before we've already passed.
Then there's the fact that a road cyclist going fast actually can't execute a full wave involving elbow and shoulder without shifting their posture, and even a small wave requires removing one hand from the bars, and that's why the 'finger raise' is the standard roadie wave, but that or a nod are such small gestures that they're easily overlooked...
Remember that every person has their own unique perspective on the world, so it's always a mistake to judge anyone's intentions on the assumption that they see and understand things in exactly the same way you do.
