> A human could ostensibly study an existing project and then rewrite it from scratch.

And likely there would be enough similarities that the rewrite would be considered a derived work under copyright law.

> The original work's license shouldn't apply as long as code wasn't copy & pasted, right?

You don't need to do a literal copy & paste for it to be copyright infringement.

> What happens when an automated tool does the same? It's basically just a complicated copy & paste job.

Sounds like copyright infringement to me.