There is a reasonable explanation. I was challenged to write an ending scene for Supernatural. Hence all the [different shots] and Chuck's narration.

...

[Aerial shot of the Impala driving down an empty, straight road with no visible beginning or end]

[we hear Chuck start talking]

Endings are hard. Any chapped-ass monkey with a keyboard can poop out a beginning, but endings are impossible. You try to tie up every loose end, but you never can. The fans are always gonna bitch. There's always gonna be holes. And since it's the ending, it's all supposed to add up to something. I'm telling you, they're a raging pain in the ass.

[Pan to Dean driving and singing while drumming on the steering wheel]

Dean still has nightmares of hell. He wakes up in a cold sweat with the sheets tangled around his legs so tight that his feet are falling asleep and a death grip on the gun under his pillow. But he gets by. He has his car and his music and his pie and his brother and his fallen angel. Sometimes, at night, when the sky is clear and the weather is mild enough, they stop in the middle of nowhere and lay on the roof of the Impala drinking beer and watching the stars. Those moments are Dean's favorites.

[Pan to Sam rolling his eyes at Dean and looking at something on his laptop]

Sam never got the white-picket fence, apple pie life he'd wanted. He never found that one perfect woman, never had 2.5 kids, never had a dog named Rex. But he did have his own little, broken family. He had an annoying brother who was probably going to die from clogged arties and an ex-angel who still had a slippery, almost non-existent grasp on human etiquette. No demon blood ever ran over his tongue again, though there were brief, fleeting instants when he missed that power. Sometimes, during the winter, Dean would make hot cocoa like he used to when Sam was a kid and the three men would sit and watch bad movies, wrapped in far too many blankets, drinking hot chocolate until they fell asleep on each other's shoulders. These moments were Sam's favorites.

[Pan to Cas looking out the window and smiling softly]

There were times when Castiel missed being an angel. He would try to spread his wings before remembering they weren't there or go to heal one of the brothers, forgetting that he couldn't anymore. But then Dean would tell a bad joke and Sam would roll his eyes and Castiel would remember that it wasn't all bad. His head may be void of Heaven's voices and his back may be wingless, but he had the Winchesters. Sometimes, when they had the occasional day off, they would park beside a river and lean up against the side of the Impala, looking out over the water and just talking. These moments were Castiel's favorites.

[fade to Chuck sitting in a small, corner side café typing on his laptop. A waitress hands him a coffee and he smiles and thanks her before taking a sip, then going back to his writing]

No doubt - endings are hard. But then again... nothing ever really ends, does it?

[He sits back, hand rubbing his chin, and smiles. He leans forward again and his hands hover over the keyboard for a second]

[Pan to Chuck's screen as he writes]

THE END

[fade to black]

[black for about five seconds]

[Still black, but we here someone typing on a keyboard and then Hey Jude starts to play. A slideshow of pictures throughout the show starts on the screen and cycles through shots/clippets from all the seasons. The slideshow ends with a shot of the Impala disappearing into the horizon on the endless road.]