With a Roots style supercharger like the 8-71 on Mad Max’s, if the supercharger isn’t spinning then there’s no path for air to enter the engine. You’d have to implement another full-size throttle body as a bypass to allow enough airflow into the engine when the supercharger isn’t rotating. SCs are very parasitic, hence their use mostly being limited to larger displacement engines that have sufficient low-end torque offset the draw. You could definitely resolve this with a clutched pulley and a bypass throttle-body, the complexity, space requirements, and engineering needed to make it work isn’t worth it. Multi-sized sequential triple turbos are clearly the superior solution to boost at any RPM.
Imagine you’re breathing through a big straw, and at the other end of the straw is a device that pumps air faster whenever you’re breathing faster, say when you’re running fast. If you turn off power to the pump, you can’t breathe through the straw anymore because the pump isn’t spinning, so you’d need a second straw that opens up only when the pump is off.
You are the engine, and the pump is the supercharger. When the engine doesn’t need to breathe fast, turning off the supercharger would conserve energy use at the expense of power output. But the design of the pump doesn’t let air bypass it when it’s off, so you’d need to engineer something (overly complex) to do it.