I agree, why Apple is doing this is an interesting and pertinent question. I don't think it's actually because they think this will put a big dent into CSAM. So the question is: what is motivating Apple?

Perhaps it's a prerequisite for deploying end-to-end encryption of iCloud Photo Library and/or iCloud Backups. The latter in particular has remained decryptable by Apple supposedly due to pressure from the FBI. Perhaps CSAM is what the FBI are using to justify their pressure.

Perhaps it's because Apple's team of lobbyists are seeing ahead to future anti-privacy, anti-encryption legislation being justified under the guise of CSAM. If Apple can show that the CSAM problem is already "solved" then such justifications disintegrate.