No one has desire for adulteration, but they have a desire for an opiate high, and are willing to accept adulteration as a side effect.

You can look to the prohibition period for historical analogies with alcohol, plenty of enterprising humans there.

Fentanyl adulterators, market creators and resellers certainly do, for higher margin selling and/or increased volume.

The traffickers looking to pack more punch into each shipment that the government fails to intercept do.

Basically it's a response to regulatory reality, little different from soy wire insulation in automobiles. I'm sure they'd love to deliver pure opium and wire rodents don't like to eat but that's just not possible while remaining in the black.