I'm sure you could recruit the same bike frame welders in China/Taiwan to make a bike for you. There are some that are willing to make custom modifications on demand for a very reasonable price. If they can do that for one-off welded bikes, then making a standardized cargo frame is absolutely doable for a reasonable budget.

Sure, there's more material, more bending, more welds, and it'll cost more, but you could certainly do that within reason. Like much of the bike industry, these markups are rooted in a foundation of bullshit.