Well, the main reason is because I don't live in the same country as the company HQs. Most countries don't have labour laws adapted to remote working yet.
But since we're on the subject, I actually prefer working as a contractor on any occasion. I pay way less taxes, have control over my retirement money, and negotiate silly perks like gym/co-working memberships into a bigger salary. FTEs in my company make 25% less (net) and cost 30% more to the company.