It seems it is happening more often than you think! Not everyone talks or writes about it!... Not often enough, as far as I am concerned, but it happens!! All it needs is a brave and adventurous couple, some "decent" (as in nice, responsible, know when "no" means "no", and "just watching" means just that) folks as watchers or participants (depending on how far you want to go), a few arrangements, and you have dogging!!! The arrangements reduce the risk, also knowing where are the safer places to go to, reduces risk significantly...
There was an advertisement on this site saying that dogging would happen there today. Arrived there and all I saw were families. I think it might be a bit of myth.