The problem is that there is no grass under these trees. It is very difficult to grow grass under trees. In a forest there are few grasses, and those that are there do not form a nice carpet like a lawn under the trees.
Trees are effective at intercepting the light needed by grass to grow.
Even though there is light under the tree, the wavelengths of light most needed by the grass have been removed. The shade of buildings does not inhibit grass growth as well as tree shade because buildings do not extract specific wavelengths from the light. Trees also intercept the rainfall and divert much of it to the dripline of the tree where it is more useful to the tree and where there may be more light to allow grass growth. Nearer the trunk of the tree, inside the dripline, water deficits also limit grass growth. For these reasons, if the tree creates a dense shade, it is best not to try to grow grass under the tree.