Cast aluminum has less strength than forged aluminum, and in particular, is MUCH LESS resistant to impacts.  If you take a hammer and whack a forged lower, you'll dent/bend it.  The same blow will shatter a cast lower into a bunch of little pieces.

The military requires forged parts because they understand that combat use (or combat training) is hard on equipment, and that the equipment is often used in ways not originally forseen by the gun designer (example: infantry soldiers will use a rifle held between two of them as a "step" to hoist a third soldier into a window of a building).  Cast lowers simply would not hold up to this abuse.

Cast lowers are fine for typical civilian range use, which really is light duty for any gun.  Take the same rifle to a shooting school, where you may whack it on a doorframe or piece of concrete during a drill, and you may well learn first-hand the value of a forged lower.

-Troy