Why study medicine and become a doctor while there are hundreds of thousands of doctors that are very good at what they are doing?
Why become a software engineer and make apps while there are apps with the same functionality exist already?
Why open up a Chinese bistro when there are many?
Why?
Because they can do it and they can still make money of it. That's why.
I wanted to disagree but given that bitcoin is decentralized, I think that it is fair to say that the analogy answers the question. There is only one problem that I can see here, is that the given analogy is not businesses which means that a competition will not be applied and we could be oversaturated with exchanges that the users will have a hard time choosing which one is to use, I am not saying that it is bad, it is in fact more beneficial because as more exchanges pops up, the better for the customers because they have options and exchanges will do anything to keep their customers.
There it is, in the end it depends on the user or consumer itself. Like Binance, it's newer than some of other exchanges, but it's a leader now.