It is legal to make liquor at home as long as you do not use a still. You might want to check with your state laws because some states limit the percentage of alcohol you can make at home. Don’t confuse alcohol percentage with proof. A 20% alcohol liquor is around 40 proof.
The reason why most people can make these kits at home is because they do not require the use of a still. Stills, like the one you saw in M*A*S*H*, are not legal in all 50 U.S. states. Yes, some countries allow using a still for personal use, but the U.S. is not one of them.