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Usually it's tied to employment regulations, funding requirements, the administrative overhead of dealing with a foreign tax code.

If you really want to make it work, open a loan out company in the jurisdiction of the enploying company. The employing company hires your loan out corporation, and your loan out corporation then pays you. That way your loan out corporation does all the work of paying and managing you in a different country. And the employing organization doesn't have to worry about any administration, overhead, legal issues. You're taking all of that on. I've seen it work. But most companies don't want to volunteer for that extra work, having a loan out is very helpful.