In many cases they've built a relationship over several years, along with the supply chains. According to them, starting from scratch here in the US would be very difficult or even unfeasible. US vendors are geared up for large volume production, and not small runs. They don't have the technical expertise the Chinese vendors do. Manufacturers do make rolling stock in the US, but locomotives for the most part are too complex to produce here.

In the middle of the 20th century brass models produced in Japan, and then Korea started the trend of importing models made by skilled craftspeople.

All this to say that it can "take a village" to produce models. :-)