It's all about cost. The assembly and test is the "low tech" part of semiconductor manufacturing and can be done well in Asia by workforces that are not as highly educated as those you need to run a fab. US and western economies have always focused on doing the highest value add portions of manufacturing domestically (where the highly educated workforce is key) while outsourcing the lower value add (lower margin) portions of the supply chain.
Consumers demand low cost. Many times they just buy the cheaper widget, because most people are not even capable of distinguishing the differences in performance or quality between two different widgets. But the difference in price is obvious to everyone and so most people just use that. The pressure of reducing every single penny out of the cost or semiconductors is enormous.
Top 10 OSAT (outsourced assembly and test) companies: https://evertiq.com/news/51003