The simpler, less cynical answer is that it's not just a US-based company, it has to be a Delaware-based company.

This is because all of the laws around founding, investing, selling, etc. a company are extremely well-trodden in Delaware, and there are very few question marks as to how some strange eventuality or disagreement might be handled.

Since startups are already incredibly difficult, this is a way of normalizing away some of the weird situations that could cause a startup to fail (and likely would never cause them to succeed), so that everyone is putting energy into the real unknown unknowns around the startup.