If you give people reservations at 7 in a limited capacity venue you reduce the number of people who can dine there. If a table is seated at 7 you can't seat anyone at that table at 6 or at 8, so you turn the table once instead of twice that night. Scheduling for two "seatings" is common practice in cruise ship specialty restaurants, especially on ships where the specialties are very popular. If you start seating people at 7, then you'll get complaints from those who can't get a reservation because you've effectively reduced the availability of tables.
Is it really that big an issue to go at 6:30 instead of 7, or at 8 instead of 7:30? Just go a few minutes late or early as is your preference.