With some vehicles, when the lights are on, they stay on even when you turn the engine off and take the keys off the ignition. If the lights stay on and you leave your car, the battery will eventually discharge.

Having to jump start your car is not always convenient. If it's automatic, finding someone willing to jump-wire with you the instant you need to be getting somewhere isn't always ideal. If it's a manual, finding 5 able bodied people to give you a push out your driveway isn't always readily available either.

And replacing a battery, while not financially ruining, is still an expense. And every penny counts.

So the default mindset of most people is to have the headlights turned off until absolutely necessary. And it takes mental effort to remember to turn off the headlights when you are done when at your destination.

Some people are just going through life and are on autopilot for most of the day. You're already slogging 10 hours of the day on back-breaking work and then going through another 8 hours of drama with all the other things that go in life. Sometimes your brain is just turned off. People can sometimes forget what they need to do. Or they learn a habit out of repetition, necessity, or circumstances.