So you would have to buy a ticket every time you go somewhere. That would work for tourists, sure, assuming there are vending machines that can print those QR codes. But it would not work for locals (imagine everyone buying tickets during a rush hour), and even for some tourists, it would be a nuisance. If you're visiting a city and you know you're going to take the subway N times, you would just buy N tickets, but with this system no such thing is possible.

And what about all those transfer discounts some cities have? Like if you're taking the subway and a bus within some timeframe, you still only pay once.