Because they're slaves kept in dorms in various South East Asian countries that can't leave, their handler wouldn't allow to go to an in person meeting or take a drug test. They're also usually lying about the city and country they're working from.
Say you have an office in South Korea. A South Korean developer starts working for you as a remote employee, and their IP looks like it's connecting from South Korea. You say "cool, awesome, but you need to come to our office in Seoul once every two months for our regular all-hands meeting," and they keep skipping out on it, claiming family emergencies or whatever. That's the warning.
That was my understanding.
This article mentions NK workers posing as Americans by paying for VPNs into people's home wifi https://abcnews.go.com/US/wireStory/fbi-thousands-remote-wor...