Tiny numbers. Long codes. One mistake.
The print is too small
Thermal slips fade fast and the digits are tiny. In a dark shop, at night, on a cracked screen — good luck.
The codes are too long
A 12 to 16 digit PIN, and a different USSD string for every network. One wrong digit and you start again.
So people hand over their phone
Every voucher, someone else types it. That is the dependency Airsnap removes.