It’s because the word migrant makes no assumption on their intention. We only know they were migrating. We have no idea if they are immigrating or returning, or visiting temporarily. It’s not politically correct, it’s literally correct.

I am a college-educated liberal. On behalf of all our kind, I hereby officially grant news outlets and “other people” lifetime permission to use the word “immigrant” whenever someone is immigrating, e.g. because they have said they are doing so.