Is there anything that isn't "interstate commerce"?
Remember, while the court was clutching their pearls over Roe v Wade and other cases not too long ago, they allowed Wickard v Filburn to stand, in which it was decided that someone growing food on their own land to feed to their own animals was "interstate commerce".
My understanding is that it's important for everything to be interstate commerce because the federal government can regulate interstate commerce, and so if we define interstate commerce to include everything, then the federal government can regulate everything.