It is probably a combination of things that makes it more difficult to remove, such as the thicker and larger diameter of the aluminum head tube. As you say, it isn't rocket science to remove the cups; it still requires brute force. But the proper tool spreads the force over the entire circumference of the cup and pushes it straight out, instead of the rocking you get with a drift punch (which tends to prevent you from removing the cup).

http://www.parktool.com/product/head-cup-remover-rt-1