I profess I don't know the first thing about bikes, wheelbarrows, machining, metals, and nearly anything involved in bringing this product to market, but...

it feels expensive, and they claim to be cheaper than dedicated cargo bikes. Is it a material/parts issue, a labor issue, or overhead? Is this class of product really so pricey? What do normal, mass-produced overseas bikes don't have that they can be sold at scale for $200-300?

One thing mass-produced bikes have is that they are mass-produced. Order 100k cargo bikes, and you will get much lower prices.

Another thing they have is less quality control. If your margins are low, you have to cut some corners. The converse isn't necessarily true, but typically it is, as high margin sellers need a good imago to keep attracting customers willing to pay their prices.

I also think their claimed $3000 for a cargo bike is a bit disingenuous. For that, one expects to get a dedicated cargo bike, with a stronger frame, better brakes, etc. such as http://triobike.com/projects/triobike-cargo-bike/

This looks more like a people carrier. Those shouldn't be that expensive.