Assembly lines give you GREAT control over where the code is located, how it's lit (consistently), and what you do on read error (can't shunt off the passenger to a read error bin, so they don't hold up the line). Rotational angle doesn't matter, perspective skew does. And then - it's a leaf of paper, so you get folds and obscured parts (yes, correctable...up to a point).
We can create live deep fakes or detect complex objects in live feeds of random webcams.
We certainly can correct a few shadows and distortions on a flat piece of paper we formatted, showing a basic symbol we designed and printed, pushed against a sensor we control, on a device we can light and shape the way we want.
NFC fails as well, you can fold the ticket just and it will break the antena.
Of course if it's a reusable ticket on a rigid medium, it won't happen. But neither for QRCode.