I worked with a guy who went on to be a trooper. A few years later I ran into him somewhere and we talked. I mentioned that I often hear the troopers start off with a routine stop for some equipment violation or speeding and then end up with an arrest for something totally unrelated. Things like headlights out, no directions, even license plate light out.
He said it's a couple of things. First a lot of them just aren't that bright. Second, since they don't care about breaking the law, they just don't care if their car is a POS. It's just a habit with them.
I don't have any independent validation of that, but it seems to make sense.