Not true. 4% (whole milk) is the hardest to froth. Above and below that percentage point it gets easier. It has more to do with a foam that is stable, keep the volume. Same reason whipping cream, which is very high fat content, is very easy to form a foam that will last a long time. Skim milk will also form large volumes of foam, but it won't have the nice creamy taste.
http://coffeegeek.com/guides/frothingguide/milk
Funny thing is, I would have thought half-and-half would have a higher fat content than whole milk, but it's usually lower around 3.5%. It just has a higher content of lactose, so it's sweeter in drinks. It mixes well with skim milk to add back the flavor and sweetness without excess fat. If you can get some heavy whipping cream it's usually around 6%. Just make sure there is no sugars or thickeners/stiffeners added.