Frisk was running away. They had no one that cared about them, so they left to find new friends. New family.

They had packed their bag and left. Their parents didn't even notice as they slipped out of the house in the night. They had no reason to think that anyone might look for them if they ran. Still, they worried about being taken back to their old life. And yet as they walked, they realized, they didn't know where to go. They didn't know where to look.

...Then they thought about Mt. Ebott.

No one ever came back from that mountain, did they? Everyone who went up that mountain disappeared. Frisk would truly have no chance of being taken back here. So they walked, for hours. Long enough that it looked like the sun might rise soon but sure enough, they found the mountain. Frisk climbed it slowly as the sun started to rise, simply glad they had packed a lot of water. They tripped at some point and hit the dirt, but they had packed for that too, and placed a band-aid over their scraped cheek. Eventually Frisk found the top of the mountain, housing a large cave with a gaping hole in the center.

Strange... shouldn't there be something here? Why did everyone go missing if there was nothing but a cave? Frisk peered into the hole, trying to see what was so special about it, but as they did so the ground gave out under them, and before they could stop it they fell into the abyss, stomach rising to their throat with fear they could only think one thing.

Oh.

That's why.