That’s true but practically speaking if you live in Europe 9/10 times you will pay 0 fees. If you don’t travel much (and during travel use ATM at the airport), you might never pay any fees.
In my experience in Spain for instance, most ATMs make you pay a fee unless you are with their bank or if your bank pays for it. In north EU a less common see fees charged.
Perhaps because you save more money on a cheaper mortgage than you spend on ATM fees, as you don't take a lot of money from the ATM because you can pay by card almost everywhere, for example?
Except those aren't related at all. You can have a mortgage at bank A and a checking account at bank B. In fact, that's the most common case for most people.
They are indeed related, it is not uncommon for banks to offer reduced mortgage rates if you have with them also other products, and often people prefer dealing with just 1 bank. So I would also contend your point of "having just a mortgage with 1 bank and the checking account elsewhere" being the most common case.
And nonetheless you were initially asking why would anyone join a bank that charged them ATM fees; the answer is that it might still make financial sense or be convenient for people. We are discussing about why there is a sizable population for which ATM withdrawal fees makes them prefer paying by card; you can of course "blame the victim" and say that it is the fault of those people not choosing the proper back. But it is just avoiding the topic, which is: this is a problem when travelling to Germany, and it I not a problem when travelling elsewhere in Europe.