Plenty of reasons stated not to make the lower out of Stainless ... but I find myself wondering why they wouldn't make one out of a glass filled polymer. You might need an aluminum insert where the buffer tube attaches, but I'm thinking that would be it.
They do make them, but from what I hear, they tend to break at the front receiver pin and sometimes have other problems. For whatever reason they don't/can't reenforce that. I can't find the weight figures either, but you don't save that much weight either.