Aluminum exhaust tubes, why not?

[2percenter]

I know this has been posted before, but as I understand it, exhaust temperatures are lower as they leave down at the collectors into the exhaust system, mufflers and then to the tail pipes. Why is that people have not explored this easy way of shedding almost 60 pounds off the bottom of the chassis by using Aluminum pipe. I've seen it in many sizes ranging from 2" to 5" at the local speed store. I know that the Turbo guys use it for their intake inlets/intercoolers and such,
I believe that the temps are not hot enough to melt the pipes from the collectors on back, unless it has to do with heat escaping too fast through that alloy.
Anyone?.