There are cars that hold less in their trunk than some characters can carry on their backs. It also slows the pacing of the game when you spend time and resources traveling to and fro to deliver things to bases or outposts rather than exploring the game. Moreso in multiplayer when you can have as much as 4x as many resources coming in at a much faster pace. I.E: 4 people clearing out one building so the clearing is faster, and they're leaving with 4x loot. That's way faster, only to be disrupted when you're instantly out of space in the car(s) you brought along. It isn't always viable to bring multiple cars either, since that burns fuel, and not everyone is meant to drive, let's be honest.