Because it will gum up in a heart beat. The G12 is not compatible at all with regular coolant.
My buddy bought some high mileage 1.8T passat, like 170K for dirt cheap. it broke a tbelt and I was the lucky one to have to replace it and put a new head on it. done even ask why. anyways, I went to flush the coolant and it was this greenish slime that came out. All the ports in the head were mostly clogged, everything coolant related you touched was slimy. Unless it popped a HG prior to him owning it and they never flushed it correctly, and then put the wrong shit in… there was def. G12 + Prestone in there and it wasn’t pretty!