Well, they sort of are. Past iterations of them... maybe not so much. Wasn't Papyrus being king already in the past so far as his consecutive timelines are concerned? He really hates to think about it.

He considers how he's going to answer that question for a while. The pressure to say it right is heavy. Eventually, he raises a claw to tiredly gesture out pictures (faint lines in the air) to illustrate his point.

"Imagine if... you go could back in time," he says, slowly. "Whenever something didn't go your way, whenever you did something you regret... you could just go back and make it so it didn't happen. So you could try again. Hours, weeks, years. It didn't matter."

"Now... think about what happens to everyone else. When you go back in time... what happens to the people who are being rewound?"