A lot of people, most even, have no idea what is happening on about 90% of their screen, about 90% of the time. And they're used to being forever in the dark about what their machine is doing and why. They expect it.
They know they're writing an email and they can see the box they're typing into, but other components, like the status bar, title bar, and attachments field, are outside their sphere of perception. At least or especially when they're not actively thinking about attaching a file intentionally, say.
It's like a serious chess player, who sees the whole board and thinks multiple moves ahead, compared to a novice who learnt the rules yesterday and can only see the horsey piece and the pieces immediately surrounding the horsey piece.
These users have a checklist approach to using their computer:
Step 1: click this
Step 2: that happens
Step 3: Did what I wanted to happen, happen?
Step 4: there is no step 4
The professor who didn't realize there's a "fullscreen" button when showing videos to the class is the same one who can't perceive the attachments field and will send a file called 'passwd'. And if the mail client puts up an alert that says, "Are you sure you want to attach and send a file called passwd to p0wn.me@gmail.com?", they'll click 'Yes' and get on with their day.