Because Whirlpool works perfect and it's the best solution we have.
Discussion
Whirlpool is obviously not perfect since I just unmixed the change.
ALL change gets mixed in WabiSabi coinjoins (unless there is a single input worth more than all other inputs combined).
Here's the WabiSabi transaction, look for yourself and try to unmix the change like I did for the Whirlpool coinjoin:
https://mempool.space/tx/01a1a055719129397fb8344b5a09e6cfe72868c8e1d750e621d8b580c96bf77b