The answer to 'why is my tree thin grass' is that the tree's canopy is blocking sunlight from reaching the grass, causing it to thin out and die. This is a common issue with mature trees that have large, dense canopies that cast heavy shade on the surrounding lawn.
The lack of sunlight prevents the grass from photosynthesizing and growing properly. Additionally, the tree's roots can compete with the grass for water and nutrients in the soil, further stressing the lawn. This can result in thin, patchy grass around the base of the tree.