> Could someone with insight into these matters please help me understand - why would TSMC or Samsung choose to build factories in the US?

It's partially to mitigate supply risk. Recently, drought has worsened the supply of chip production, many of which are concentrated in Taiwan[1]. If you go on any PC parts forum, you will see that parts like new GPUs are nearly impossible to get. Some years back, flooding in Thailand[2] also threatened the supply of hard drives.

[1] https://thediplomat.com/2021/03/taiwan-prays-for-rain-as-dro...

[2] https://www.theguardian.com/technology/2011/oct/25/thailand-...