Earlier quoted context omitted.
Because it isn't about CSAM, IMO. You see this with plenty of social issues, notably firearms ownership. The claim is we will restrict/license/outlaw xyz for the kids, but really, a data-drive approach would have focused on different things entirely (eg additional behavioral health services for kids, suicide prevention etc) The same technology/access used for CSAM identification can find copyrighted files, materials…
The thing is that a society does not always do only ONE thing. You mentioned that there are better ways to protect kids. But maybe they cost more (e.g. banning firearms is dead cheap, while setting up a structure with lots of psychologists specialized in teenager problems is really expensive). Also you can do one and the other ... at the same time. E.g. suicides of kids/teenagers ( And keep in mind, the total deaths…
A note about copying - you have to look at the culture to see if how things will work. Germany has vastly different socioeconomics than the US, and socioeconomics are major factors in crime and homicide. Not to mention the percentage of people who own guns and are pro-gun are vastly different. Applying the same regulations in the US will take decades for criminals to no longer have many guns and then they will switch to knives and screwdrivers as we see in the UK, which has about dowluble the violent crime rate as the US after accounting for differences in the stats (somewhat culturally/socioeconomically similar). So it might make the gun numbers better, but it won't solve the real drivers.
The best approach would be to address the underlying factors and causes of all violence while still allowing freedoms for the 44% who do not support stricter regulation.