> It's not hard to imagine the partially legitimate reason that on occasion, someone will drive with the trunk open

No, it's a much more serious and likely reason -- people stopping on a highway at night, getting out, and opening their trunk for some reason (like a spare tire, fluids, etc)?-- then their lights (and the reflectors in the lamp housings) are pointed at the sky.

Or, movable bodywork is more prone to be misaligned during normal operation.

Headlights get out of alignment sometimes. I posit that likelihood goes up if the lights are themselves mounted on a hood/door/whatever that can also go out of alignment.

My dad, in the 1960s, put reflective tape on the rear bumpers.