Is that really the reason most planes still use carbs? I have never heard of fuel purity being an issue. Cars have had FI for many years. I fly a FI plane and would never ever buy one with a carb. I thought that carbs, with their lethal carb ice issues, are still used only due to tradition / certification inertia.
You are probably right that the main reason why carbs are used is because the old engines still in use happen to have carbs. But I am rather certain carbs have the advantage of being less susceptible to fuel contamination than fuel injection systems, since the latter have much more narrow passages internally that could be blocked by contaminants. That is one reason I have heard for why Cessna went from 3 fuel draining points to 13 (!) when they fitted a fuel injected engine in the C172.