Cheaper labor. Everyone else went, and if you want to compete (or survive), you follow. Also it costs money to keep up, billion dollar upgrades means few can afford their own fabs. Somehow it became better to partner with a foundry that served multiple companies "over there". Perhaps as things shifted from bipolar to CMOS, easier process with steps and less proprietary technology for all I know, that made it easier for companies to merge into using the same technology.Your chip example, how important it is and all…. Why were all those fabs moved overseas? Were those flying the red, white, and blue buying the US made chip vs the dumped overseas chip 25 years ago when all that industry went overseas?
Once screw pitch got standardized, then anyone could sell a screw that could be used by everyone, but perversely, I bet there were less companies making screws. The only way to make money was through volume. I wonder if something similar happened, once you go mostly digital content, it's all just gates and connections. Analog requires transistor matching, resistors and capacitors (which eat space), high voltage handling.
I don't know who's left in USA. Polar Semiconductor is, and they proudly state they are in the USA (although Sanken still owns a piece). China wants to buy from China as it builds its economy. Big economy (that we no doubt funded) so no small incentive for US based companies to pursue.
I suspect some portion is environmental--some of the manufacturing steps were not so environmentally friendly years ago, and that no doubt helped spur the movement. I suspect it's cleaner now but when stateside had to clean up their act, it may have been just cheaper to offload for all I know.