Problem is that this means the images have to be kept around in order to compare them. So, often these caches of child porn and other non-consensual images which often are poorly secured are targets of hacking and thus end up allowing the images to spread more rather than less. And the people sharing these things don’t usually use the services that do this kind of scanning. So in general, it has more negative than positive effect. Instead, education to prevent abuse and support for the abused would be a better use of the money spent ln these things. But more difficult to profit from that and it doesn’t support a surveillance state.

It shouldnt be google and it should be the companies hosting the content in question. Why trust a third party especially if it’s google. They probably have AI scan that image regardless so they will definitely own it.