They get less light (consider that in summer they only get sin(theta) light compared with cos(theta) light for a horizontal garden, require much more infrastructure (you need to hold the plants up, get water to them, complex equipment to pick food).

I've built many over the years, they are nice to look at, they protect the walls from the weather, they can cool the house (though much less efficiently than putting reflective foil on the wall) and they are fun to make.

But they're not going to save the world.  Sorry about that.