because we're all voyeurs, it's bringing people watching to your screen, it's the same reason soap operas have always been so popular.
I think it's too easy to write them all off as entertainment for the braindead, it has to be more than that to find basic interaction between a handful of people so appealling.
Compare, say, TOWIE to Downton Abbey, it's not the acting that's of interest in DA, it's essentially a show about a slice of life you haven't encountered based in the past, TOWIE is simply a slice of life some people may have encountered based in the present. Would the show with which more people could associate not be expected to be the more popular?
Just to clarify, I don't watch TOWIE or BB (by choice), but I did see Downton Abbey on Sunday..well part of it, it bored me so I dozed while the lass enjoyed it.