The key (from Apple's POV) is that this is done on your device, so the model can be audited, and users will know if it changes or is suddenly enabled where it wasn't before. Apple has documented the entire threat model and their design decisions realted to each threat vector.
It's worth reading the document, as it becomes pretty clear that this is a step towards enabling E2E for iCloud Photos.
The alternative to what Apple did is cloud-based scanning, which is less transparent, permanently disallows E2EE, and is more vulnerable to being changed by national decree. If CSAM scanning is going to (or is already) mandatory, I vastly prefer Apple's method here.
[1] https://www.apple.com/child-safety/pdf/Security_Threat_Model...