Sheer laziness. The same reason that drivers do not slow down for signs that read "hidden intersection" or "hidden driveways" or "school bus stop" (at the times when buses are out) or for obscured railway crossings. 99 times out of 100 they get away with it and no one is affected or gets hurt.
And that 1% of the time when something happens it is everyone's fault but their own.
Reminds me of a story of a former co worker told me. He is a brakeman for Canadian Pacific and was running five box cars over to an industrial siding at 5mph across a four lane road with a 70 km/h speed limit. The crossing had crossbucks but no lights or gates (a flag man is used here) as it was infrequently used. A woman in her car screamed up to the crossing, failed to see the locomotive and the front end of her vehicle hit the loco and the rest of the car flipped up and smacked the cab of the locomotive. She hit so hard she actually shifted the railway tracks. As she was being checked out by the paramedics she screamed at the engineer about she was going to sue. She was brought up short with retort that, au contraire, we're going after you for the damage to the locomotive, the tracks, and the fines for being late in delivering the box cars.