I can create an account on the machine without having root access. Which gives me a shell. Which means I can exploit a flaw to change root password. Which means

I can create an account on the machine without having root access. Which gives me a shell. Which means I can exploit a flaw to change root password. Which means I have access to cookies and other fun things. Which means I can probably access that person's gmail which is pretty much game over, man. Bedroom analogy is wrong. The computer is the castle. They have it. The problem is that your lord knows the secret entrance to your main castle (in the cloud). You want one of two things to happen: Keep the lord safe, or kill the lord so his secrets are gone with him. The attacker formatting the machine, or replacing the hard drive is fine, he already physically has it, just like he can take the vault and throw it outside and make a new one. However you don't want him accessing that vault.