Honestly, it’s 9pm so unless the store is 24 hours (and even if it is) then they’ll be trying to close down a clean up and get things ready for the morning shift which starts early.
When I worked at McDs years ago a few big orders deciding to sit in the restaurant around 9pm could mean the difference between getting to go home at 12:30 and getting an OK nights sleep vs getting to go home at 2:30 and getting a terrible nights sleep before they might have to come in at 10am the next day.
You could argue that if they didn’t want customers at that time then they shouldn’t be open - which I would agree with - but obviously the low level grunts making your food don’t get to make those decisions.