Because the US doesn't have the supply lines for that. You simply can't compete with a Shenzhen where there are a dozen PCB factories and a warehouse for virtually every part you can imagine within a 30-minute drive.

In Shenzhen you can have Some Guy in a truck doing a parts pickup round around town a couple of times a day. In the US you're waiting for overnight Fedex from Digikey. And sure, there's a lot you can stock in advance, but virtually every board will have this one weird part you don't stock yet.