They have optimized idle performance for when the display is still on. Which is nice, but configuring the screen to switch off when idle will save far more power.

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.