Most of the climate benefits from "tree-planting" really derive from reforestation. Forest ecologies are complex and take decades-centuries to develop to the point where they can function as meaningful carbon sinks. The cycle was covered in my biology 101 class in college, can't find a graphic covering the overall cycle at the moment, but the point is it's not just lots of trees = carbon sink, it's the enormous community of life that develops around (and also often precedes) the trees that makes forests -- not just "groups of trees" or even "a rather large group of trees" -- function as carbon sinks.
It's a fantastic idea for the long-term considering all the other problems we're running into with global deforestation anyways, but businesses whittled the concept down to plant tree = good because that's easier than dealing with the causes of global deforestation (hint: businesses)