Longeyes, a good 1911 can be made out of plastic, but the problem is that no one is happy to just copy a receiver in polymer. They all want to add this or that, changing the feel and look of the weapon...which is exactly what attracts most people.
As to weight, I doubt seriously that you will even notice. Even a single-stack poly-frame isn't that light. I carry a stainless S&W 686 and three reloads on my duty belt, every day, and don't see how it is cumbersome or bad on the back. You get used to the weight.
Another advantage to the weight is that it can steady you when firing. A bud at work had a shoulder injury that gives him a serious wobble in the iso or weaver stance (18"@25yd is wonderful for him). He carries a glock, but loved how the weight of my SA 1911 slowed his shake and allowed him to tighten the groups.
What's the weight difference between a poly frame and an aluminum one?