Because you really don't control them, they do. Anytime an exchange can close down or won't let you have access to them. I remember Poloniex would let me get my tokens out for 2 months without any reasons :)
You also have to consider that exchanges can be hacked. They have been hacked in the past, and thousands of people have lost their money. What most people refer to is Mt Gox, which you can read about here: https://en.wikipedia.org/wiki/Mt._Gox
Because you really don't control them, they do. Anytime an exchange can close down or won't let you have access to them. I remember Poloniex would let me get my tokens out for 2 months without any reasons :)
Exchanges I believe is the most insecure way of keeping your cryptos.
You also have to consider that exchanges can be hacked. They have been hacked in the past, and thousands of people have lost their money. What most people refer to is Mt Gox, which you can read about here: https://en.wikipedia.org/wiki/Mt._Gox