Because they really want to prevent child abuse? Because they want to offer technology to governments like China that can then look for anti-Chinese-government content?
It's pretty likely that the vast majority of people involved with anti-CSAM efforts are in it for precisely that reason. Apple is just an organization of people, after all. For every free speech absolutist that believes child porn is a necessary evil we must accept in order to preserve our freedom of expression, there are probably a thousand people who are comfortable with scanning hashes if it stands a good chance of catching the assholes making it and spreading it around.
Announcing end-to-end encryption the same day suggests this program was blocking end-to-end encryption from moving forward, too.