This isn't entirely true. In much of the Western world – outside the US – CSAM (or equivalent legal terms such as "child pornography", "child abuse material", "child exploitation material", etc) can include text, drawings, AI-generated imagery, etc, which no child was abused to create, indeed the child depicted in it may be entirely fictional. Canadian law goes so far as to treat material which "advocates or counsels" the commission of CSA as CSAM - which the Supreme Court of Canada insisted (in the 2001 case of R v Sharpe) doesn't include mere political advocacy for its legalisation (as in e.g. the NAMBLA Bulletin), although some will question whether that insistence actually cashes out in practice.
Now, as a father of school-aged children, I find the whole idea of groups like NAMBLA rather abhorrent – but, I'm hesitant about the government making it illegal for people to express abhorrent ideas, because there are likely ideas which you or I hold which somebody out there considers abhorrent.