I see your point of view and you are right, I totally agree. But your answer has nothing to do with the original question.
I asked that why do we need bitcoin mixers, if there is Monero. We can change BTC to XMR and vica versa on no-KYC exchanges, and make an XMR to XMR transaction between the changes. This is a theoretical question.
It could be that users of Bitcoin mixers wanted a much more convenient way of changing the image of their Bitcoin holdings instead of using the tricky BTC-XMR-BTC way or they are just making sure things will work accurate or else their funds will be compromised most specially if it was from a "dirty money".