Because it's not about the handling, it's about the way it looks, the way you feel when you hear it or see it even if you're not driving it, if you are lucky enough to drive it then it's the feeling when you turn the key, it's the high maintenance that doesn't matter to you, it's the way others move out your way or drive up to you and talk to you because of what you're in. I see a tuned car I think that's nice, I see a supercar and it's like wow, speechless and so is everyone else around. You hear people who don't know about cars getting the name wrong like "is that a Ferrari" when it's a Lamborghini and such.
The Countach was a supercar but you know what it was a terrible car but to the many rich and famous who bought it that didn't matter as it looked like it should have missles firing out the front, wings out the side and a jet engine from the back, it looked crazy and mental at the same time, it made people expel expletives when it drove by them and stopped people and traffic when it drove by. In fact to be honest a supercar is hardly ever a good car! :D