I assume there’s nothing to stop you from running a second fuse box if you actually need it… just as long as the cable runs and input to each are sufficient for all the loads (consult your electrical expert).
Only a guess, but I assume that’s much the same reason most manufacturers don’t make boxes with more than 12 fuses… because the studs, circuits, etc need to be built to withstand the maximum voltage of all fuses (worst case scenario) within it.
As it stands, a 12 fuse box usually has a 100amp max… with would be HUGE in this type of build. 