Cost, and also practicality. Most of the "green building" plans that you see come from artists with no knowledge of construction or engineering working in 3D programs where everything is weightless, and all building materials are free and have infinite strength.

Trees are heavy. Assuming a fairly mature tree with a 10-inch diameter trunk and around 20 feet of height, the wood alone weighs around 800 pounds, and that's before you consider the soil that the tree sits in (100 lbs per cubic foot, so at least a ton for the whole planter) or the water that it needs (100 gallons per week ~800 pounds)