Cash handling isn't free! You have to pay someone's time to count + reconcile + deposit it, or If you're dealing at any volume, you'll pay an armored car service to collect it. There's inevitably "shrinkage", or else business processes (more time and more overhead) to avoid it.

Cash is king for hiding transactions and avoiding taxes. If that's the situation then I won't say you don't deserve a cut, but for rules-following merchants taking cash isn't any cheaper than paying the credit card fees.