I wouldn't mind these machine so much if they always accepted cash. The problem is that more often than not they don't.
When I was on vacation in London about 5 years ago when Visa had an outage: https://www.theguardian.com/world/live/2018/jun/01/visa-outa...
I was trying to pick up some food at the only grocery market that was within walking distance of where I was staying. The grocery store only had self-checkout machines. As the checkout line piled up, the payment system failed to process my card. I gave it another try with another card, and after a long wait that one went through. By that time as I was looking around, I noticed that pretty much everyone else around me was stuck at the machines and weren't moving on. I assumed there was some sort of outage going on.
I guess I came within a few seconds of needing to scramble to try to find a way not to go hungry that night. I had plenty of cash on me, but that wouldn't have helped me complete the purchase because the machines simply didn't accept cash. There was no obvious way to get an employee's attention so I could give them my money. They were probably stuck with some of the other customers scratching their heads as to why the magic card charging system, which appeared to have been the only possible way to pay for food at that grocery store, wasn't working.