You're confusing idle (the user is not present) with idle (the screen contents is not changing). The latter is extremely common when someone is e.g. reading a document. Nothing changes for a minute or two, the user scrolls down and in an instant the contents changes, then nothing changes for a minute or two again.

You don't want the hardware in a high-power state during the time it's not doing anything, even when the user is actively looking at the screen.