In the era before smartphones, barcode readers were highly tuned devices that emitted a specific frequency of light and the sensor was tuned for exactly that. Cellphone screens generally don't emit that frequency of red, and I guess absorb rather than reflect light sent to it, making the screen opaque.

So even if you, the human, can read it doesnt mean the device that authorizes you can.

A lot of 1D barcode scanners cannot read phone screens reliably.