Eh, as a third culture kid with a US passport and 50%+ friends being international/TCK - it's honestly because the VISA process is appalling.
Out of a sample of 10 international students I went to school with at Stanford 20 years ago (I just eyeballed my FB friend list and took 10, totally biased anec-data I know):
• 4 are now citizens - all their green card processes took 7+ years, except the one person who got it via marriage
• 2 are STILL in the green card process
• 1 intentionally went home for opportunity
• 3 left due to VISA expiration issues. 2 went to Europe, 1 went to Canada. 2 are CS grads, 1 has a PhD in ML.
It massively sucks. We're talking about amazingly resourceful people who got raised at whatever sheep farm in Kenya and Uzbekistan, made it all the way to Stanford via merit-based scholarships and what not, and then got ejected from the US because national policy... and now some other country with friendlier VISA policies got 'em.