In 2020 Brazil for example, you are not allowed in theory to obtain a double nationality which you were not allowed to have as a birth right.
This means that Brazilian students e.g. in France, many of whom could easily obtain the French citizenship had to either chose between:
  • giving up their Brazilian citizenship. Who the fuck would do that? Brazilians love their country despite all!
  • not getting French citizenship. This meant in France having to come 6 AM once a year to some police station on some suburbia to stamp a piece of paper, plus having your VISA completely dependant on your employer for several years until you could obtain a permanent VISA, making it very hard to change jobs, and putting you in a constantly precarious position
  • keeping both citizenships, ignoring Brazilian law, which is extremely unlikely to sue you anyways for this bullshit law, and just hope for the best
Can you guess which option Brazilian students would usually pick?
As a poor country, you have to allow people to obtain multiple citizenship. Students are not going to go back because they don't have the foreign citizenship. They are just going to have to ensure shittier jobs for a few years, thus diminishing the chances that they will actually lean anything useful to bring back to your country later on.