Here probably the part.
In China, probably the labor.
Doubtful, either would be more cost effective. Lower labor costs would yield lower product/distribution costs for the resistors, so that $3 resistor in the US may be a small fraction of that in China. Evidence that this is the case, is that China still uses automated manufacturing for electronics...
Just a gesss. I bet you can test the parts at a workbench and bin them prior to soldering, assuming soldering doesn't change the value. The saving would add up very quickly at $3 a part saved if you are doing thousands.
I believe your assumptions are faulty. One, I don't think you appreciate how much human labor costs, even in places like China. Cheap electronics exist because they can be assembled mostly/completely in an automated fashion.
Two, you can almost certainly not manually bin the parts for automated assembly. Indeed that is one ways hobbyists get cheap smd parts, leftovers from those large spools used in automated assembly. If it was cost effective, they would be reused.
I guess it depends on the scale and what kinds of products actually need this precision. If it's only 6.5 digit DMMs then I agree with you, the quantites will just never be that high.
Well, the demands for those itens are almost certainly reasonably high, since they can be purchased via non-specialized dealers like Digikey. They wouldn't be getting manufactured unless there was sufficient demand. And most likely you need them even in devices with relatively high error rates. The issue arises from cumulative error, which is almost always higher then (and can be much higher) than the error margins on the individual components.