You can only see photons when they bounce off something and into your eye. So you have no way to see the photons as they travel towards and through the slit, only after they hit a wall on the other side and reflect back to you.

So there’s no way for you to observe the photons with your eyes before they’ve gone through the slit. In order to observe them as they head to the slit you need to hit the photons with something to measure where they are, and it’s this interaction that collapses the waveform and makes the light travel though a single slit of the two.