I agree with you to a point. Using the term "custom build" to sell an AR makes me chuckle. With that said, Guntroll also made some good points in the OP. It doesn't take any true gun smithing skills to put one of them together. The guys on the line at Smith and Wesson don't have magic fingers. Installing a headspaced barrel assembly on an AR requires more muscle than anything else. As long as you get it past minimum torque, it either works or it doesn't.
I have seen a few "expensive" rifles for sale on TGO that were listed for less than the bill of materials. Parts quality can vary widely, but receivers are probably the exception. Those $50 Palmetto State lowers look great, and are quite possibly made on the same tooling as the M&P lowers. The difference in quality is all in the BCG, barrel, and furniture. It's just the way a modular platform works. Nothing is proprietary, no matter who "manufactures" the final product.