Bets like the one discussed here are a rhetorical tic of the "rationalist" community.

> ...parent's argument sounds reasonable to me.

Kinda sorta. It's just a weird ritual of theirs to demonstrate commitment, and they often have a really hard time understanding that their weird rituals and mores are not universal and they can't reasonably go around expecting random people on the internet to follow them. Challenging someone to a bet like this comes off exactly as a challenge to prove your commitment by putting a bucket on your head and banging it with a stick for an hour, just because that's some weird thing the challenger's friends do among themselves.