First, very nice nitpicking of irrelevant systems that just happen to have stupid DC distributions. Try using GURPS vs D&D5e next time. Second: You fundamentally misunderstand what people mean when they say “swingy”. 3d6 is objectively less “swingy” than a d20 because the extremes of the range are going to be much rarer, which is what people mean by “swingy”. They’re not talking about whether they succeed more often or not. A 50% chance is the same on any kind of dice.