To answer your question directly, GM uses junk metal. They like to have a pre-made "spider" of LR, RR brake lines, fuel send, return, and EVAP all under one clip. So they use mild steel to save a buck or two.
For every brake line I've had to replace, I've also done fuel-- usually with nylon.
Stainless is harder to work.
GM builds a lot of complex things as part of the vehicle and engine. They could fabricate the SS brake lines if they chose to.