Because there are LARGE numbers of illegal aliens with false drivers licences or no licence at all causing accidents, often fatal ones. (They aren't required to take a driving test when purchasing a false licence.)
Here on the California Central Coast, an illegal immigrant mecca, there are several accidents a day that are bad enough to make the evening news. Our insurance rates are higher. I have to drive super defensively--three days ago I had to pull over to the shoulder to avoid a Mexican passing in a no passing zone headed right at me. ETC.
Wow! Did you stop to ask where he was from? And his legal status?
Of course not, since you probably got a 1/10th of second look at his face and nothing more.