Or, there's risk to being in their home country where academic freedom might not really be a thing.

it's like why if you show serious promise in soccer at a young age, you go to Europe as soon as you can - you will be better developed there in a more mature environment as opposed to, say, the USA where you can only get decent coaching in a few major cities, and even then the gulf between the coaching at a top Spanish or English club versus an American one is huge. Or if you show promise in tennis at a young age, you get your ass to Florida as soon as you are able to.

Which is indeed a benefit for their countries.

And is a loss for the US.

The brain drain was real and the US was the beneficiary and that may be ending soon.

Not sure what your point is? Are you happy that the US will be worse off than it was before?