That depends on many factors of course, but I've seen it ballparked around 2-4%. You need safes, cash registers, insurance, etc.

But credit cards in the US are also like 3% and come with additional risks.

It takes time to do those things. Time is money. It takes time to reconcile in a double entry accounting system the cash you received in business operations. Software does it automatically.

Where I live there is also a bank fee for dealing with large amounts of cash.