Cephalopod precursors evolved eyes and then brains developed from the eye so their eye is structured correctly.

Vertebrate precursors evolved the brian first and the eye evolved out the brain as a sensory stub. So it's upside down and inside out like in picture.

The nerve cluster goes through the back of the eye splits and folds back to end in light receptors. Light hase to go through the nerves before hitting the sensor.

There is even a reflective layer after the sensors that gives the sensors a second chance at picking up the light. This is what causes the red eye or green eye you sometimes see in flash photography.

It would require a genetic rebuild to fix this and the intermediate steps evolution usually use would be so disadvantageous they are selected against. So the right combo of mutations to give us a working octopus eye is VERY unlikely to happen.