Ok, I understand your question now,
The reason is simple,
when I started this project, I wanted to make something simple as possible,
except if you add the PCB feet, the top and bottom shell are the same
(just assembled in mirror position), also the front and back panel,
in order to export one of each element, and print them twice.
Another advantage is,
when you print the shells, you can print them in different colors, and mix/switch them as you want.
In one word, "modularity".