I'm no economist, so take this with a grain of salt; but my guess is that it comes down to economic differences between the products.
Intel, AMD, ATI, NVDIA all design and produce high end integrated semiconductors. Designing and fabbing chips requires a fairly small number of skilled workers and some massively expensive hardware(Chip fabs are in the billions of dollars these days). Places like north america (Intel) and Germany (AMD) are good for Fabs; because those cost a fortune no matter where you build them, so it makes sense to put them near concentrations of highly skilled workers, universities, etc. Chip packaging and similar ancillary tasks are usually outsourced to other countries(malysia packages a huge number of chips).
Manufacturing motherboards, 3d cards, cases, barebones, etc. requires a small number of skilled EEs, and Tiawan definitely has their share; but also requires lots and lots of labor and manufacturing capacity. This makes nearby China a big asset.
Thus, we have the current situation, where high end semiconductors are designed in the US, Europe, and Japan, by residents of those countries and by talented expats from India, Taiwan, China, etc. Often fabbed on location for things like microprocessors, or fabbed in Taiwan or China for some things and packaged in malaysia.
The Taiwanese OEMs then design the boards that will carry the chips(or work from reference designs, depending on what segment of the market we are dealing with) and do the fabrication in China.