Not all instances keep logs...
Some intentionally discard them, look into the policys of the instance you're signing up to.
All your comments and posts are hosted on your parent instance then shared to the federated instances.
Some instances don't even let you sign up with an email or make it optional.
Lemmy.world keeps logs, but much more controversial instances often don't.
Also it's much more complex, because you have to think about the scope of the potential lawsuit as well as the given evidence that a user is actually sharing the material infringing on their copyright; which will not be a large amount of the user base. They can't simply sue a user for having an account, the user has to actively be sharing infringing material.