Mushrooms in the MCU (Mario Cinematic Universe) are entities of pure energy. Ingesting them breaks down the energy molecules to elongate limbs, organs, and skeletal structure to considerably large but fragile proportions that revert to their original state when hostile organisms come into contact. When kart racing, those same mushrooms are shoved into a special compartment wherein a machine breaks them down into pure energy, resulting in a brief but large boost in the machine's maximum speed. It also results in a devastating pollutant that, with each boost, brings the Mushroom Kingdom one step closer to total climate annihilation. But since it's the sport of choice for many including the royalty's princess herself (and the kingdom's most lucrative activity monetarily-speaking), the side effects are summarily ignored.
It's quite simple, really.
