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.
There is one manufacturer that uses a insert around the RE, not sure if it's NFA or who. The poly lowers save about 8oz in weight
A SS lower would be of benefit on a bench gun with the added weight. I think the cost would hamper sales.