I may be one of these people. At least for the more obscure places, the highway pullouts and national forests and things, if I see another person parked there, I’ll typically park next to them. Safety in numbers, the more people parked in a turnout, the more legitimate it looks to park there
Not very up on biology, so not sure if this would even be a thing, but I would say some kind of internal structure like plants allowing animals to overcome the square-cube law