He's definitely not aboveground, the body would wash out on the first hard rain! That's really interesting, I hadn't thought about why it's done that way because I take it so much for granted. It's pretty resource-extensive when you think about it because you need extra dirt (and therefore work) to build up the mound on top of filling up the hole. The coffin would displace some dirt, sure, but I'm pretty sure it's not nearly enough for that big mound.

I guess the mound is basically another marker in addition to the gravestone to show that it's there. It's no doubt a status thing, too, because the bigger deal a person is the bigger their mound--see here for a royal grave of a king from the 6-7th century A.D. and scroll down to see it scaled against people. The thing is twice the height of a grown person (a usual mound comes up to a grownup's waist or chest at best) and must have taken enormous work compared to a normal grave. Humans and their status symbols, eh?