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.
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.