Leftover ancient instinct to seek protection in new/strange places by clinging together. A modern expression that addresses this: safety in numbers.
It certainly reveals who is operating off of instinct and who is thinking for himself or herself.
Agree too that there are a lot of campers who are ignorant of common camping courtesy. If someone is parked away from others, then it should be a sign they want more quiet and privacy. Respecting campsite boundaries by not walking through them is also a given with the exception of emergencies.