The photon (as a field excitation) goes through both slits, but is quantized so only has enough energy to trigger a mark at 1 spot on the photo paper.
> Why do we not still have to consider interference in outcomes after the photon makes its mark on the paper?
If we want to be completely accurate, we should. However so many interactions happen so quickly that the law of large numbers quickly takes over and obfuscates the quantum reality. Technical term for this is decoherence.
> Why are quantum computers hard?
Exactly because of this decoherence. It is very difficult to keep the qubit state isolated from the environment throughout the computation.