It doesn't though.
The most effective way to shut these forums down is to register bot accounts scraping links to the clearnet direct-download sites hosting the material and then reporting every single one.
If everything posted to these forums is deleted within a couple of days, their popularity would falter. And victims much prefer having their footage deleted than letting it stay up for years to catch a handful of site admins.
Frankly, I couldn't care less about punishing the people hosting these sites. It's an endless game of cat and mouse and will never be fast enough to meaningfully slow down the spread of CSAM.
Also, these sites don't produce CSAM themselves. They just spread it - most of the CSAM exists already and isn't made specifically for distribution.