And there is a risk to make my BTC's dirty.
All Bitcoins are equal be design, there is no such thing as "dirty" Bitcoins.
If I am right: Why do people use BTC mixer?
Privacy and convenience. Going through an altcoin, especially on a decentralized exchange, is much more work.
Isn't changing BTC into another crypto and change them back, not the much better solution to cut off the trace?
It depends who you're trying to hide your transactions from. Or your balances. You don't want your local coffee shop to know you own a fortune in Bitcoin, and may not want them to know you gamble. If that's the reason, any exchange will suffice, even though chain analysis companies may still find the connection.