The way that I'm looking at this situation, from the state's perspective, the purpose of the state's education system is to improve the quality of life of its citizens.
We are aware of all the systemic issues that plague every level of Nigerian society. Issues that are going to take generations to fix.
In the meantime, how do you make incremental change?
Citizens who receive public education owe a debt to their society.
It doesn't make sense, at least from the state's perspective, to allow citizens who take advantage of public educational resources, to leave the country, regardless of the state that the country is in, so that the country is not able to reap the benefits that it's public educational system is supposed to yield for the populace. Only for another country to reap the benefits.
Everyone wants Nigeria to be better. A win for Nigeria is a win for black people globally.
The way I see it, Nigeria can try to improve on the underlying issues, which will take generations, while continuing it's brain drain while it takes those steps.
Or curve the brain drain, and make incremental steps to address the issues.
It seems to me, the people of Nigeria will see more of an immediate benefit from the second option than the first. To be sure, the first option is ideal, but it doesn't appear to be manageable at the moment.