There are 2 reasons I see now:
1. Living cost ratio. Well, you might get 10x salary comparing to Asian developer, but living cost is not at the same rate. 100x is different story, but again related to living cost, you might jump in there, when you get there you might end up with 101x living cost.
2. Assume every knowledgable person will leave their homes for 10x salary, 7x living cost and they are earning 3x of money and country will end up with idiots, slow development, slow financial growth in whole country. It takes some time for developing countries to be in decent place if professionals do not leave, it takes forever to be in good place if everyone will leave just for 3x or maybe 10x.