I think the wave/acknowledgement goes back to the days of the uncivilized past. Anytime someone came onto one's property or was passed along a route it was there to determine friend or foe. Anyone who grew up or has spent time in a rural setting before cell phones know what I am describing. Go out into a rural area and all of the local drivers wave to each other. They even have their own custom waves. These people depend on each other in their everyday lives. As for cycling, that person you pass may be the one you have to depend on for help later up the road. So you might wave to let them know that you are a friend and not an enemy.