A company can move countries without actually changing its physical location. Tim Cook would stay right where he is.
Apple has physical offices all around the world. The country of incorporation is a legal detail and has pretty much no physical significance.
Also, you ask what's wrong with bringing manufacturing back to the USA?
It's one thing if Tim Cook proposes it voluntarily (although it would impose a substantial cost penalty that competitors in South Korea, China and Japan don't face).
It's something else entirely if the president of the federal government forces them to do it. Last time I checked, the law didn't grant the president that sort of power and the US is built on the concept of freedom.