Cash is riskier, yeah. But it’s also a lot cheaper depending on what the services and machines cost.

No, I don’t think this is true, and most merchants are coming around to this view. Adding an extra 15 minutes to the cashier’s shift counting cash, adding an hour to the manager’s shift driving the cash to the bank, doing all sorts of analog counting processes, maintaining a secure chain of custody so that the cash doesn’t get lost or stolen, the risk of actual violent robbery, it’s all going to cost more than the 3% fee that the processor charges.

but you have to keep the receipts from those as well just like you need to keep cash

No, everyone’s POS systems are totally digitized. There’s a database with all the transactions, not a pile of paper receipts. And the database actually links each transaction to an actual distinct card payment, rather than a digital note that the cashier took that much cash and put it into the register.