I think Western academia is mostly reliant on immigration. Few natives would accept the low pay/high workload combination that is a postdoc, but for an immigrant from a third world country it might be an amazing opportunity. Same reason most fruit and berry pickers also are immigrants.
Don't even know where to begin...but according to this [1] less than 5% of students in the US are international students. 1 - https://www.fwd.us/news/international-students/
https://www.nytimes.com/2017/11/03/education/edlife/american...
Note that the definition of "international student" excludes those with permanent visas. The number of foreign born PhD students is therefore significantly higher than 64%. Don't know if there are any stats for citizen/non-citizen share of PhDs though.