I think they’d need some hardware to verify that the transaction happened. A small business owner can use their own phone, but other businesses need a smartphone per point of sale, which might be more expensive than a card reader (that one likely has to have anyway)
You can use a phone (or whatever) to verify the transaction, but in practice I've never seen anyone do that. It probably helps that crime is pretty low in this part of Asia.
For a shop to adopt QR payments, all they need a printout of a QR code. No hardware, not leasing some card reader, zero maintenance.