I have pondered over this a while and refrained from mentioning it in public for fear of ridicule, but my son had an idea that just won't go away.

He reckons long barrows [Kits Coty was the inspiration] were only built that way so it meant placing the capstone was easy because it was just a ramp. It kind of makes perfect sense to me, that the lifting of a capstone would be no easy task and that dragging up a purpose-built ramp would just make the job relatively simple. It obviously doesn't work for a lot of them, and of course there are earthen barrows to consider, but if not the actual reason for their construction, I am beginning to wonder if that was how they got the capstone up there.

Ridicule away!