When it comes to disk encryption, at least in the home, the threat model isn't somebody sitting around in your home finding a way to exfiltrate the currently-unlocked filesystem; It's someone taking the computer or the drive with them and leaving.

In your analogy, the key atop the vault vanishes as soon as the vault is moved from its location (loses power).

It makes sense temporarily. You can always move the key to your pocket later if nobody steals it.

I mean, I assume you'd set the unlock-on-reboot flag, and then immediately reboot — at which point the unlock-on-reboot flag gets automatically unset.

So, sure, it's a bit like leaving the key on top of the safe... while you have the safe open. Which isn't all that odd.