Probably because they are also adding a feature to parental controls which when enabled checks images sent to the phones of your children 13 and under and if the image matches a known bad image they give the child the option to accept it or not, warning the child that if they elect to accept it their parents will be notified and given a copy of the image.

That has to be done on the phone because Messages is end-to-end encrypted. If they are going to have to have hash matching on the phone anyway for that, it makes sense to also use that for checking images that are to be sent to the cloud.