This is the biggest reason I’d say. There would be little area for joining the peices. They hold shoes so it’s not like it wouldn’t be structural but it’s also added time to fit and calculate to make it efficient where just slapping them on till they fit is quick and easy. Extra material yes, saved time also yes.
The inset idea gave me a idea though. You could make some awesome hidden drawers in these things. I’m thinking of making a few of the shelves for my office actually!
Oh the things I could make with that much ply!
/patiently waits for @Zachs Ply with killer shipping rates/