Let's make the world more fair

In humanitarian relief and development settings, there are often international nonprofits providing services: housing, healthcare, food, energy, etc. Those nonprofits are usually staffed by a combination of locals and foreigners. From what I hear, the foreigners are very likely earning more than the locals for similar work in the same setting. Why?

Seriously, why?

Because it costs more to live in the foreign country (where the foreigner will return after the humanitarian response ends)?
Because the cost of the foreigner's education was higher?
Because the market price for the foreigner's skill set is higher?
Because the foreigner has higher expectations for their earnings?

That foreign humanitarian providing services probably sees the situation as unfair -- I am making more money for the same work here than my local counterpart. Shouldn't I negotiate with the local humanitarian worker so that we make the same amount of money? Wouldn't that be fair? 

Here's an unrelated video: