The rocks I know about are real, but they didn’t naturally occur where you see them. They were deposited there as part of the landscaping.
The most notable ones seem to have been placed to discourage cars from cutting a corner over grass.
Is this really about truly fake rocks, like hollow fiberglass or plastic shells? I’m aware they exist but AFAIK I’ve never seen one. Of course, a well-made fake rock would fool me at seeing distance, so maybe I have and didn’t know it.
They are often used to hide above ground utility devices, like gas meters, cable boxes, water anti-siphon valves, etc. It’s typically done for aesthetic purposes since a large rock makes the area look more natural even if it’s a fake rock.
I’m in the foothills of the Appalachians and one or more (very roughly) roundish 4-ish foot boulders in spots near roads and at the edge of parking lots in front of businesses are pretty common. I don’t know if any of them are fake, I’ve always taken them for granite.