A simple answer would be because it is a "sports car." The bold designs and speed of these cars are highly sought after. They target young men who want to be flashy and older men in a 'mid-life crisis'. As to why they are so expensive is usually because of low production numbers and performance.
In my opinion buying a brand new sports car isn't worth the money. I can easily turn any vehicle into a fast machine. People will always want the next best thing and the next best thing will cost more just because manufactures know that people will pay for them.