Why are grafts generally kept above ground?

I've seen some posts on other forums that suggest that planting the graft below ground may allow the tree to sprout from the scion wood and get on it's own roots, instead of being on a root stock.

Now if you are on a dwarfing root stock, I can see wanting to avoid it.

But in some cases I've heard of graft incompatibility that can take years to present, or you have a root stock that tends to be short lived (E.g. grafting plum on nanking cherry). Would overplanting the tree above the graft allow it to develop onto it's own roots?

 

Here it depends. Roses for instance are grafted mostly at the root crown.

 

Putting the graft below soil level leaves it at higher risk from soil-borne diseases, so more of the grafts will fail. Also, not all plants will produce new roots from the scion wood; some would even be killed by being buried deeper.