Can't hackers recover your steam account?

Why would the contributor do that?

More to the point, has this ever actually happened to anyone?

Steam Support will not award an entire account based on a picture of a serial from one game contained on the account.

Hell, even shitty EA support won't award an entire account based on just one game.

Isn't account access reacquired through the combination of username, pass, and email?

They can't take your account, however, they can take the game.
If they create a support ticket and provide a picture of the key (with the ticket number written on it) and the receipt of payment, the game will be transfered to their account. Once i had to do the same, when i bought a retail copy of a game from a store (brand new), and the key was already activated on steam.
Anyway, most keys on this site are from bundles, and i think those are safe.