There is a lot of reason but the main reason is to make money. Don't think of it really hard since there are more exchanges coming from the next following years. They all want to make money andtake advantage of the continuous increase in numbers of altcoins in the market. By that reason, it is enough for them to make new exchanges.
There's some truth to that. Unfortunately, the main reason for initiate a new exchange is not to make it better or safer. NO, it is a money making project. Sure, you invest some initial money and time into it, enough to make it last until you get enough customers, get some money and ditch.
So, I'm yet to understand why we need any more exchanges when we already have the likes of Binance and Bitfinex.