As has been mentioned elsewhere, a 1942 Supreme Court case, (Wickard vs. Filburn) greatly increased the power that Congress could exercise in pursuit of regulating "interstate commerce". Pretty much any activity, by anyone, anywhere, could be put under "interstate commerce". Literally growing your own vegetables on your own farm could now be regulated by Congress, on the assumption that if you were growing those vegetables yourself, you wouldn't be buying them, and would so affect the market price for said vegetables.

Yes, the Supreme Court really did rule that way.