Software Hiring: Remote, but US-only. Why?

I'm simply astonished by this reticence of US companies that are looking to hire software developers to look for people abroad. I've seen countless ads that say "100% Remote", but need to be US-based. Which I simply don't understand. Why do you care, if the work is fully remote, if the person is based in US or not? Please explain to me: why is this a thing? Note 1: if the person is from another country, you can simply contract them via their own company, or something similar, which for you as a company, is simply a LOT easier in all aspects. Note 2: I do understand if you have some big Intellectual Property, and therefore, you'd prefer local people. But my assumption is that very few companies are truly in this boat, and obviously, you can always sign an NDA.