The potential to carry a lot of passengers means very little when a vast majority of the cars on the road carry only one person. How often do you see 7 people all wanting to go to the same place at the same time?
Z3 @ 27 mpg with two = 54 passenger miles per gallon.
SUV @ 11 mpg with five (never happens) = 55 passenger mpg.
Worse yet, all those who insist on commuting in an empty P/U. Most owners drive those things thinking they might need the carrying capacity someday.