What if every store accepted both cash and cards?

I'd be fine with this as long as you don't have to pay a fee if you're not paying cash (which is often a thing for small shops).

Unfortunately this is not the world we're living in :) it's often both cash and cards OR just cash. Berlin is a really good example of that, you always need cash on you which tricks the many tourists who come from countries were cash is not really a thing anymore.

The credit card companies make the merchant pay a fee for using credit cards - so it's not the shop's fault. That's another reason why cash is alive in America.

The bigger reason is to avoid taxes. If it’s paid with card, taxes have to be paid, and with cash, the merchant can make an extra 30%+ by not declaring it.