The CFAA is for unauthorized access to a computer system. Downloading from libgen isn't unauthorized access that can be prosecuted under the CFAA.

Say you have a network. You want to keep someone out of it. The tool for that is the CFAA.

If you have a book, and you want to stop someone from reproducing it, the tool for that is copyright.

They cover different domains. If you're not breaking into a network, you're unlikely to be charged under the CFAA.