There are loads of Canadians [0] on TNs and H1Bs in the US and most don't particularly care about ever getting a green card. How do you explain their presence if you believe companies only recruit foreigners from poor countries in order to exploit their desire for US residency?
[0] https://techvibes.com/2012/02/24/there-are-350000-canadians-...
Even very high Indian salary would not buy that US lifestyle. Of course many people will like to couch that fact as having friends' circle in US, kids schooling etc makes it difficult to relocate.
I suspect 80%+ of H1B visas go to Indian/Chinese nationals simply because they represent a similarly large proportion of STEM graduates interested in emigrating from their home countries.
TN you can do it, but it's messier.
Similarly for a few other occupations. And, of course, some want to hedge their bets in case they do want a green card in the future.