While there is logic in people hanging out together based on their activity. But in this example both groups ride bikes, it is the same activity. Therefor, the real reason is elitist attitudes and group exclusivity. It would be the same as not hanging out with someone because their skin color is different or because they use nvidia video cards instead of ati video cards. All of these are actual trivial differences compared to the actual activity, just like the type of bike you are riding. All bikes can drive down a road and unless one of the bikes has a serious performance issue, there is no real differentiating factor. Elitist car drivers do this too, with their little bmw or insert car maker here meetups. But just because these are elitist, it doesn't necessarily make them negative, a lot of people find these groups simply by looking for others who own something similar to them. And while they could just go for a ride with a group of buddies who all own different cars, for some reason they feel a part of something if it is a bunch of people in similar bmws.