I have a plastic hinged box that holds an emergency sewing kit. There are two hinges in the back, the kind that snap together with a plastic tab fitting in between two plastic beads (for lack of a better term).
On the left side, the outer part of the hinge is connected to the lid.
On the right side, the outer part of the hinge is connected to the bottom.
I remember seeing lots of plastic boxes made this way.
I assume you mean a box like this. (it’s in my desk at work)
Both halves are the same dimensionally so only one type of mold had to be designed, documented, and machined. They then inject a different material to made the 2 pieces.
As with all plastic stuff, because it was cheapest effective method to do it.