Here are a couple of reasons why you cannot leave an event and re-enter with the same ticket:
- Venues want to reduce staff after the beginning of the game. Rechecking everyone for contraband would require more staff
- Venues want to prevent people leaving an event and passing their tickets on to people who did not pay to enter an event
- Venues want to prevent people from leaving the event and finding hard alcoholic beverages in their cars, consuming said beverages and re-entering and thus causing trouble.