Because any simulated CSAM laws have been, to my knowledge, all struck down when challenged.
To the best of my knowledge, calling drawn works obscene has been upheld in courts, most often because the artist(s) lack the financial ability to fight the charges effectively. The artist for the underground comic “Boiled Angel” had his conviction for obscenity upheld–most CSAM work falls under obscenity laws–and ended up giving up the fight to clear his name.