A drive-in does not make its money on the ticket. Admission covers the print and the power; the snack bar is what keeps the gate open through another winter. That is the honest reason outside food and coolers are not allowed past the gate - not a rule for its own sake, but the difference between staying open and being one more empty field.
Popcorn is popped fresh in small batches through the night rather than piled up at dusk, so a bag bought during the second film tastes like a bag bought during the first. Walk up between films - the line is shortest right at the changeover.
A rolling cart also passes through the rows twice a night for anyone who would rather not leave the car - flag it down with your parking lights, never your headlights.
