Because logic

I'm not sure what sort of queue this is for, but it may be just for check in or bag drops. Even if it for the same flight, it's better to keep them separated. COVID doesn't have a wide dispersal range. That means if someone has it on the plane, only a few rows around them will be affected. But if people were bunched together in the queue, and then ended up sitting in different spots in the plane, they end up spreading it to more people than if the queue was distanced