The answer to 'why is my drainage system thin grass' is that a poorly functioning drainage system is a common cause of thin, patchy grass. Improper drainage can lead to waterlogged soil, which prevents oxygen from reaching the grass roots and causes them to weaken and die off.
Some common symptoms of drainage issues include standing water, muddy soil, and moss or algae growth. To fix the problem, you may need to improve the grading and slope of your yard, install or repair drainage pipes, or add drainage features like French drains or dry wells. In some cases, the grass may need to be reseeded or resodded after the drainage problem is resolved.