It's also because people are swapping their headlights out with something non-standard to what the car shipped with.

So then you end up with people putting high-end halogen, xenon, or LED bulbs in the headlights, and they're not realizing that the light profile for these is different and requires the headlights themselves (the casing, not the bulbs) to be realigned for optimal spread.

If the upgrade is done professionally (at a legit shop, not Tom's backyard oil change) they should be taking care of that as part of the service.