Because those types of groups tend to really suck. For one, grouping is social, and you can never say a thing. I've found that the people that tend to burn through stuff like that are jerks, especially if things go wrong, and they have an attitude of "you guys suck, and all this great stuff is happening for you because of me". Don't forget what was already mentioned about looting chests. So just pass on the experience, and the type of person that flocks to it. Been there, done that.