IDK but why? Because at 6:45pm at an intersection, the LED streetlights in the intersection turn on so late, but near dark. Why is this?

It could possibly be the photocell they're using, or how it's oriented. It wouldn't be because of the light source being LED itself.

The setting for HID has to count for their warmup time, so they have to be turned ON earlier than the light is actually needed.
That means the photocells have to be set to higher light level.
With LED you don't have the warmup delay, so the setting may go exactly when the light is needed.
Plus the photocells use to have a time delay (needed for HID to prevent dark hot restart times when something flashes onto the cell)  which they have to count on too, the difference could become significant.
The result was many HIDs were ON when the natural light was still 10x brighter, with instant start/restart source this mode would be just waste of energy and lifetime...

The type of photocell used matters.  For the new LED fixtures installed here to replace the HPS, the LEDs come on later and goes off earlier.  I like it.