We can speculate. But I bet the fact that the supply chains needed by a US plant stretch across the globe (mostly back to Taiwan, Japan, and Germany) has something to do with that. In Taiwan, supply chains are local. Things like wafers might not be produced in the US yet.

You can hop in a car and visit them. In the US they are across the Pacific and in a very different/inconvenient timezone. It's a 15 hour gap. 9 am in Arizona would be midnight in Taiwan. And there's the anti meridian running through that so it's a day later over there as well. And the business days barely overlap.

I bet all that adds some friction in day to day operations. Lost time, shipping delays, miscommunication, etc. There are solutions to this, of course. But I'm sure that adds complexity to an already complex business. So, limiting that overhead to just 5-20% sounds pretty good to me.