What you're missing is it effects their profit model - if you can bring in a seemingly infinite supply of food and drink from your vehicle the vendors can't push their wares. This is often disguised using security theatre, where they say they don't allow this because people might smuggle items in by making multiple trips. However they could just as easily re-search you on entry, which makes this an invalid concern. They'd also have to post entry staff for longer to re-admit people, but could just as easily confine re-entry to a specific window. Again, money.
