Probably because is an ethnicity and nationality. There are ethnic Chinese people all over the world and a few countries and regions are made of a majority of ethnic Chinese but are not related to China. Calling them the same thing is playing into the PRC's "all ethnic Chinese pledge their allegiance to China" nonsense.
Perhaps but I haven't encountered that myself. I'm ethnic Chinese that's a citizen of another ethnic Chinese majority nation so I've encountered this specific type a lot more.
So, Chinese scientists?
Probably because is an ethnicity and nationality. There are ethnic Chinese people all over the world and a few countries and regions are made of a majority of ethnic Chinese but are not related to China. Calling them the same thing is playing into the PRC's "all ethnic Chinese pledge their allegiance to China" nonsense.
Isn't that true for every (older) country though?
Perhaps but I haven't encountered that myself. I'm ethnic Chinese that's a citizen of another ethnic Chinese majority nation so I've encountered this specific type a lot more.