I rode a 50cc scooter almost 40 years ago. Back then motorcycles were very much in the minority. Every biker waved to the other. Even Hells angles would wave to me on my little nerdy bike.

I noticed that as the popularity of motorcycling increased the waving decreased. There is a tendency of people in a small minority to band together. As the group gets larger the bonding decreases.

I have noticed that if I am walking in my city early in the morning and there are few people, when someone passes, he/she says "good morning". Later, when it is more crowded, no one says hi.