The aircraft is also much faster and more maneuverable with a vehicle inside, than swinging underneath. Think "exposure to ground fire". That's why the Marines kept the M151 in service long after the Humvee came into the inventory. We could carry a couple of jeeps inside our CH-53's (yes, we called M151's jeeps, small j) quite easily, then blast along right over the trees.
Also, remember that a lot of things like putting a Mule inside a Huey were done purely for fun and for the photo. Remember turning someone's VW or Pinto sideways in a parking spot when you were in high school? (Make that horse/chariot/wagon for some of us older guys, Fiat or equivalent for the younger folk).
Cheers