Fair enough, but put a 70-pound pack on a weightlifter or a bi-athlete, and he'll go farther and faster than a marathoner with the same pack. Marathoners are amazingly "fit", but in a light-weight, low-drag way. Every extra ounce is something they (understandably) hate, so they don't carry, and therefore aren't capable of carrying, much weight. That's the nature of marathoning.
A greyhound can run fast & far, but they don't use greyhounds for emergency work (rescuing lost mountain climbers, guarding facilities, etc); they use larger, stronger animals like St. Bernards and German Shepherds.
Same principles apply regardless of species. To sprint "out of dodge", the un-burdened marathoner certainly has an advantage; but to actually accomplish something once you get "out of dodge" (and have the means to do so), takes more than just low-drag speed.
If speed were the only issue, it wouldn't make sense to carry anything at all. If speed didn't matter at all, it would make sense to carry a boatload of crap. As much as I dislike the way the word "compromise" is often used, that's exactly what we're talking about here; much like the CAR-15 itself.
An FAL unquestionably hits harder than the CAR, but IMO, the CAR hits "hard enough", so that's what I carry; hence the compromise. If power was the only issue that mattered, the FAL (or whatever heavier caliber gun) would win out. If paring down weight was the "only" issue that mattered, then no gun at all would be carried. A compromise.