Not everyone wants to drive around in little nimble 4-cylinder hatchbacks, or have the space constraints that people in Manhattan have that dictate small cars (if they do cars at all).
Usually, if you're buying a minivan, it's because the extra space is NEEDED. A family with two young children requires the space for hauling around stuff like diapers, baby wipes, toys, playpen, and so on. Or, they're using it to haul home some flat-pack furniture from IKEA, or hitting the Costco to stock up on groceries. Or, they're hauling home the ENTIRE youth soccer team from the meet that's 3 hours from home - stuff like that.
Then, of course, we have the people who equate size with safety, and don't care that it's costing more in fuel or makes it a chore to park because they can get hit broadside by a semi-truck and walk away.
Oh, and the Zip car idea doesn't work so hot in, say, Jackson, MI. What they do have is a local Enterprise that'll happily stick you $200 a day for a minivan rental, or the Home Depot with the $20/hour pickup truck.
Life at the end of the cul-de-sac comes with entirely different priorities.