I'm trying to figure out if this is a regional thing, some sort of recent linguistic shift, or if I'm just imagining it, but I literally never heard it until moving to Florida around 3 years ago and now it's everywhere, sometimes even when I travel. It used to always be just "welcome."
Edit: Apparently I wasn't clear enough, the question is about the phrase "Welcome in" rather than just "welcome." Not whether they say anything at all.