But actually a lot don't, and when you try to pay, even contactless, with a non-Girocard, it won't work.
Visa and Mastercard don't magically let you on their network because you have a card machine.
The bigger mystery is why stores in some other countries are happy to have a payment method chosen by the customer eat into their profit.
In particular, I was staying in Nordhausen for work for a couple weeks. The hotel I stayed at tried to refuse to accept that I paid by card online in advance. Every small restaurant I went to refused to accept either a credit or debit card, and only a couple chain (read:overpriced) restaurants would accept Mastercard. When I tried to buy an Ethernet cable at a general store they refused to accept anything other than cash once they realized I was a foreigner.
Maybe it’s a myth in large population centres, but definitely not overall. I had roughly the same experience throughout most of Thuringia, with even gas stations in certain areas only accepting cash.
Not sure where you people live, but this myth has to stop, eventually. I can literally count on one hand the n. of shops NOT accepting cards - it's most of the times shops which, I guess, use cash to have flexibility in their accountings :)