They did similar studies to determine the looks of an ideal romantic partner. The resulting look was very close to the average of all available romantic partners. However, that doesn't mean that individuals don't have different preferences. It is the same if you do a study of people's favourite colours. The average result will have little to do with people's personal preferences.
These designs are not people's preferred designs, just those that everyone least dislikes.
It's well known that symmetry is considered beauty, so when you average up enough imperfect faces, you'll get a symmetrical and thus beautiful average.
These designs are not people's preferred designs, just those that everyone least dislikes.