Why is it that Villagers only plant Wheat? 

That is, if I manually plant a village's gardens with everything under the sun the Villagers will harvest it all up but then only plant Wheat to replace it.  So, after sufficient some time goes by the only thing growing in any garden within a Village is Wheat.

The Villagers need to plant whatever is available in the game - or - at the very least they should be able to plant whatever they are harvesting once the Village is created.