Bitcoin mixing vs Bitcoin exchange.

Hi. I can't help think that bitcoin mixing services have become completely useless, and surpassed, thanks to the rise of over 40+ BTC exchanges.

When you make an account on a BTC exchange, they give you an address only fo you, that you can deposit money into.
A short while after your deposit into that address, you will see that your address has been cleaned out, with all the btc sent to several other addresses, that are presumably the exchange's own addresses/cold addresses/addresses of other users withdrawing from the exchange/etc..
So while your ADDRESS itself is empty, you are still owner of X btc deposited into your account of that exchange. Thus when you withdraw, you will receive btc that is completely untainted from the first btc address you used to deposit it.

Thus the question, why the hell are bitcoin mixing services still exist? With some that even cost a fee? Do people still even usse them? Or am I completely mislead in considering BTC exchanges as bitcoin mixers?

EDIT : I apologize if this has been posted in Btc discussion > Legal. I meant to post in in the Discussion subforum. Sorry about this.