Because there is not enough Greed art that I am aware of. This is sort of an old picture (and by old I mean it sat around for a week or so before I bothered to scan/color it), and the concept is out-of-character, since I seriously doubt Gluttony would be able to sit around grinning when he's only a few feet away from Greed, and Greed and Envy would probably be ripping each other's heads off. But, is it really so wrong to think GreedxEnvy would be the second-best HomonculusxHomonculus pairing ever? (only GreedxLust is greater, in case you were wondering).
(And yes, according to several of my friends, it IS wrong to think that. ._.)