The use of having different coins is that not every coin is going to function the same. You have improvements on financial, healthcare, gaming, payment, privacy, and the list could go on forever, Couple that with the fact that all coins can be made differently and from that stems healthy competition. I don't know why we can't have an exchange like that but to me it seems a complicated endeavor and perhaps this is why?
First of all not all the coin are the same, in some occasions they might be used for different purposes. Second, we can say same about money and shares, why we are exchange like them? It is complicated but it is currencies of the future.
