You can't go into a store with a gun and demand the cash out of the register if there is no cash.

The actual cost is shrinkage from general human accounting mistakes and all the extra time it takes to manage.

I worked at the gym in college and we sold like one item a day and it was still a whole bunch of work and pain to keep up on the cash counts correct.

I definitely believe that all businesses should take cash as much as is reasonable, but logistically it is understandable why some choose not to

You shouldn't do that anyway; also, you can't skim a credit card I'm not using/carrying. There are crime arguments on both sides.