Then supports wouldn't be very useful in any lane (not counting mid lane BS). ADCS scale with items more than most champions, so they need to farm a lot. Supports are meant to protect them as them farm, and they soak up experience from the ADC.
Mages, tanks, fighters, and even assassins scale better with levels than ADC's. So they need as much experience as possible. It's the same reason why you leave the jungler right before they kill gromp/krugs when leashing. They need the experience and if you stay then they won't level.
Plus you'd be turning the jungler into a much heavier role as they'd have to protect squishy alone ADC's as well as help other lanes.
The support role is needed and there are people that find it fun. You shouldn't think the game would be better by outright removing them.