Cost and I'm sure there's some sort of legality in people expecting lights to automatically come on as unfortunately certain people in the world will not do it themselves if they're meant to be done for them. 

First of all. All new cars have DRLs - daytime running lamps. So there's always lights on. And some cars do have sensors that will auto on the lights when it's dark enough. Like if your driving around dusk. Usually only fancy cars. But a lot of cars have sun load sensors on the dash for use in the AC system maybe? 

ELI5, how is it possible to drive a new car at night with the headlights off?