Take a look at the STEM faculty in any top school in the US. A decent proportion will be foreigners who came to the US to do a Masters/PHD and now produce cutting research, all thanks to them being allowed to study in this country.

The issue with completing your education outside the US and then coming in is that it usually limits you to just industry. If you want to get into academia, you’ll need some kind US credentials or it doesn’t work.

And banning these people from US academia will very seriously damage US competitiveness because such people are in short supply everywhere in the world, meaning they could produce that cutting edge research elsewhere if the US bans them.

Because US education is incomparably better than the majority of where the foreign students come from

There was an analysis done a couple of years ago that found out that the most elite chinese, russian or indian computer science student at their countries version of an Ivy League, are quite a bit worse than the -average- computer science student in the US.