It is a big decision to pack up and leave your home forever, which is why most people dont attempt it. 

While there are academic theories for the brain drain issue, they are far from simple and you can find points to support both arguments.


[hider=Rethinking the Brain Drain -Oded Stark, World Development Volume 32, Issue 1, January 2004]
[i]When productivity is fostered by both the individual’s human capital and by the average level of human capital in the economy, individuals under invest in human capital. A strictly positive probability of migration to a richer country, by raising both the level of human capital formed by optimizing individuals in the home country and the average level of human capital of nonmigrants in the country, can enhance welfare and nudge the economy toward the social optimum. Under a well-controlled restrictive migration policy, the welfare of all workers is higher than in the absence of this policy.[/i]
[/hider]

Also when you look at brain drain alone you are ignoring the positive effects of diaspora's, remittances, investment, trade ect.


[hider=Migration's New Payoff- Devesh Kapur and John McHale Foreign Policy No. 139 (Nov. - Dec., 2003)]
Every day, migrant working in rich countries send money to their families in the developing world. It’s just a few hundred dollars here, a few hundred dollars there. But last year, these remittances added up to $80 billion, outstripping foreign aid and ranking as one of the biggest sources of foreign exchange for poor countries. Following a boom in the 1990’s, this flow of money is lifting entire countries out of poverty, creating new financial channels, and reshaping international politics.
[/hider]


It is a complex issue, which doesn't play well in our fake news world.