That's how surge pricing works.

Right now they have to set a price that's reasonable both when the place is empty, and when it is jam packed. The price they're using is going to be right in the middle.

If they can change prices dynamically, they'll want to slightly reduce the crowd when things are busiest, and earn more per customer, and they'll want to draw people in when it's empty. When it's empty they have to pay for workers to sit around doing nothing. They make more money (or lose less money) if those workers are kept busy. To do that, they'll want to increase traffic by lowering prices.

They have zero incentive to make it cheaper, ever.

Sure they do, competing against other burger joints. There's a reason the prices are set where they are now: they're high enough to generate a profit, but low enough to entice people to come there instead of other burger joints. But, right now those prices are static, not dynamic.

A burger joint that's open late has to pay for staff. If they're just sitting around without any customers they're losing money. If they can lower prices to get people to come to them instead of Mc Donalds or Burger King at night, then they can presumably make more money.