In my experience, some of them do have transport shuttles (golf carts, small vans, sometimes busses) that just do loops of the parking lot. Even some of the stadiums that don't provide that as an intentional house service find groups forming small shuttle companies to opportunistically make a few dollars on that service (and jam up parking traffic that little bit more because there aren't designed service routes, just these ad hoc third party providers trying to make a quick buck on a game day).