S, that's a very good point!
A cabinet showroom recently opened, and they carefully re-mounted the existing wall receptacles inside the cabinets. That is, they took the devices out, made exacting holes in the cabinet backs, then mounted the devices to the cabinet back.
This is a violation of the 'zero clearance' required if the surface is combustible. The issue can be resolved with the use of those plastic-sleeve box extensions.
Naturally, this is not an issue when the entire box is brought forward and mounted to the cabinet ... or when a deep mud ring projects out from the wall enough to enter the cabinet.
