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.
We dont need lots but they do keep on building and making one yet they do know that this is a profitable business when you do really get some sufficient users of your platform
which would be the most or main challenge on any new platforms out there.They dont get enough users until they do die and vanish and the cycle continue.As long this market
do still have money to involved on and do able to make profits with it then they wont be stopping on making one.This is a big profitable business when you do able to get some
sufficient marketshare but same as you said that we arent needing lots if there are already some existing which do already make the job done well.