Why isn't the "remember me" checkbox in login forms enabled by default?

On the majority of sites, a "remember me" checkbox on login forms is not selected by default. Most users would like to stay logged in after login. It annoys users each time to have to enter username and password just because they forget to check that checkbox.

Why not just enable the "remember me" checkbox by default?

If the user does not want to be remembered, he can just uncheck the checkbox.

Update: "remember me" checkbox in Gmail login forms is not selected by default, but if the user logs in and enables it, this choice is saved to cookies and the checkbox will be enabled by default at the next login.

Update 2: by 37signals:

Has the time come to kill the “Remember me” check box and just assume that people using shared computers will simply logout?

Update 3: I wrote a WordPress plugin which enables "remember me" by default. So you may use it if you want to.

Update 4: "remember me" is enabled by default on Basecamp login page.

Update 5: "remember me" is enabled by default on WordPress.com login page.