But if that is related to interstate commerce, how can anything be unrelated to interstate commerce? Any kind of transaction or activity is liable to have an impact that can't be limited to one state or one locality.

For example all kinds of things might make a place more or less attractive to visitors or people thinking of moving, and thta clealry can have an effect on "interstate commerce". Does that mean that the Federal Government should have the right to tell people across your country what colour they should paint their doors? Or how to dress?