Why is one mounting screw left behind each electrical panel's cover?
If I am seeing your photos right, it's a mild security feature. When I worked in electrical contracting, that was done so that a panel could have a lock installed on the door, and then (theoretically) prevent unauthorized access by surreptitiously removing the panel front.

Like most locks, they are meant to deter rather than completely prevent access. If someone wants to get in, they will, but the unauthorized access will take some effort and will be detected.