Why do web designers assume if we hit the button which says "exit" or "logout" we've done so accidentally?

I don't know anyone who either randomly clicks all around a web page because they don't know what to do or whose hand slips, moving the mouse over the exit button, and in their confusion, they click "exit".

Luckily for those a pop up message will appear saying, "Are you sure you want to exit?" - for the rest of us, I guess we'll have to continue, each time thinking to ourselves, "well why else would I have clicked the button?"