They are trying to prevent CSAM images from being stored and distributed using Apple products. If that goal is easily circumvented, the whole motivation for this (anti-)feature becomes invalid.
I a way, this architecture could potentially even make Apple products more attractive for CSAM distributors, since they now have a known way to fly under the radar (something that is arguably harder/riskier on other image sharing platforms, where the matching happens server-side).
One reasonable strategy Apple could have against that is through constantly finetuning the NeuralHash algorithm to hopefully catch more and more offenders. If that works reasonably well, it might deter criminals from their platform because an image that flies under the radar now might not fly under the radar in the future.
NB. I'm not trying to say Apple is doing the right thing here, especially since the above arguments put the efficacy of this architecture under scrutiny.