I guess it is because no other mat has overlapping coils. In the apple one they wanted you to not care where you put down your devices. Since they have so many coils, it then becomes likely that the 3 active ones overlap.
I suspect that engineering does have a design that meets regulations but they couldn't get the cost down. I.e. the extra circuitry in both the met and possibly the devices to precisely sense which coil(S) are providing optimal charging is cost prohibitive.