Well, this is how the market works. There is a competition, like everywhere. Exchanges are just business, every transaction or registration is beneficial. I'm using CoinDeal but every trader has his own favourite one. There is a broad variety, it gaves you an opportunity to choose the best coins' prices, fees etc. I think that competition is good in that way, you always have a choice.
We also don't want to give 1 exchange a lot of control. If everyone was using binance only, the amount of bitcoins they would hold would be insane, they could literally control the market if they wanted to, a possible hack would also be devastating, it could almost end bitcoin.