"Illegal alien" is a term used throughout federal immigration law, e.g. https://www.law.cornell.edu/uscode/text/8/1366

That's just some kind of language bifurcation by political affiliation in the US. The right-wing term for immigrants who didn't enter the US through ports with documentation is indeed "illegals", though it seems the word "immigrants" is as pejorative among that crowd. The left-wing term for immigrants, both domestic and international and irrespective of arrival method is "transplant" and is fairly pejorative as well. I don't think "transplant" is any more specifically humanizing.

It's just that documentation-status matters to one, and does not matter to the other.