1. Labour-wise probably 3-4 times the Taiwan, but labour cost of fabs is microscopic in comparison to everything else.

Taiwan does not have dramatically lower taxes than US, but there is something particular to how working capital is accounted which will make for a double digit difference in the in TW vs. US.

Third is supplies. Even before the semi moving to Asia, US fabs had to import a big portion of their supplies.

2. > Why did the US outsourced the semiconductor fabrication in the first place

Why did US outsource almost everything, even when it makes no sense?

It simply tough, tedious, problematic doing business in the US.

I wrote about it many times before here.

You mind your own business, very much literally. Few month down the line some trouble comes: lawsuit, taxmen, utilities, creditors, SEC, city hall, suppliers, random activists, labour union... pick any.

As for 2), it is less about outsourcing and more about gradual loss of competitiveness. The scientific and technological development between 2000 and 2020 led to a lot of corporations, not just in the U.S., falling out of the race because they could not keep up the pace. Nowadays, only Samsung and TSMC are left, with Intel lagging behind, but still not completely out.

Similar consolidations have happened in the past and in other industrial fields. This time, two of the three surviving champions are simply not American.

In general its a dirty process. There are a lot of toxic chemicals involved. People might get sick. Margins aren't as good as software and its very capital intensive so investors would rather invest in software. Lots of booms and bust in the semiconductor industry.