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)

Floor plate loading is 50-250lbs/sqft over the entire area, with hotspots far greater. If what you really wanted was trees you could put them in there. I think he may have meant more just an open green area.

Yes, just a cost issue. The park floors is a really good idea, especially for very high density city developments.

I had some ideas in similar thrust to yours, I think it's worth following up on.

One of the cooler things you could do is space the floor plates so that you give people berths in a building. They then install walls and possibly a subfloor and run utilities. If they want to leave the space open they can, if they want to cut it up into rooms they can.

My building has this, and so do two others in my neighborhood in the unfashionable East Bay. These are all buildings that are 20+ floors. There’s a outdoor space on the top “parking” decks. Then the actual towers are set back on a smaller floor plate. Then there’s outdoor space on the roof deck of the tower.

Not sure what the value to residents in shaded mid-floor outdoor space, vs having sun-filled decks in places that make structural sense. Mid floor parks are just an aesthetic embellishment for people outside the building.