Genuine question: is labor cost really that important for cutting-edge fabs? I would have assumed the tooling would be far more expensive.

I am doubtful as well. We are told these factories cost 10s of billions to build though the number of people on the floor doesn't seem very different than any other factory. I find that hard to believe that labour is a primary driver.

If you read a couple of articles featuring what TSMC said, then it's clear that work culture/ethic is a major reason why they have so much trouble getting their Arizona plant running. This work ethic is at least in some part related to cost: they expect long, possibly unpaid, overtime.

Yes, but you amortize the cost of the tooling out over the lifetime of the tooling, and you're paying on a loan for that. Labor is an ongoing, and increasing cost, not just wages, but also insurance, payroll taxes, etc.

Also remember the tooling is a fixed cost, it'll cost you (many) millions in Taiwan the same as it will in Japan or the US, so the cost of skilled labor is what ultimately affects your margins.

Could provide subsidies like we do with farming.

Are Taiwanese labour costs significantly lower than Japan these days?

Taiwan educated generations of people into this field, Japan probably has higher costs to hire in the same sector or simply doesn't have the people.