So, Mixers provide service of shuffling bitcoins or provide addresses loaded with bitcoins along with private keys. In short, they provide service so to create anonymity between sender's and receiver's address so that no one can trace the transaction.
My question is why not use Exchanges? When we deposit funds on exchange, bitcoins get deposited on exchange's cold wallet. But when we withdraw funds, withdraw gets processed from exchange's hot wallet. So in short, it is very easy to do anonymous transfer by depositing funds on exchange and withdrawing to other addresses. Why to pay 1-3% fees on Mixers when it could be done for much less on Exchanges. Exchanges like Binance charge just 0.0005 BTC to process withdrawals.
Once I have wondered why not to send (anonymously) bitcoins through the chain of a few exchanges to achieve effect of their shuffling and the answer I have been contented with was the following:
...
these exchanges can and will still keep track of the coins,
...
.. apart from it costing a lot of $$$, and exchanges who probably won't be too happy about it.
Obviously, trading on every of those exchanges would refine the mixing but the matter of the cost of such solution remains.