I've dabbled in this area as well. The problem seems to be that metal shops in the US are too small-scale and don't have services to help walk customers through manufacturing. They expect you to know everything, and just tell them exactly where to punch, bend, and cut stuff.
Chinese factories don't have this problem because they're super used to dealing with unfamiliar customers. Unfortunately, there simply isn't enough demand for most US manufacturing to support such services. It's a chicken-and-egg problem. Not enough customers to afford baller customer service, and no good customer service to attract more customers.
There are some US companies that can do some of this. For example, SendCutSend is an all-online sheet metal fabrication company. By being online-first, they can get enough orders to support Design Services for customers. Unfortunately, they're still not large-scale enough to help with assembly.