The answer to 'why is my drainage system thin grass' is that poor drainage is a common cause of thin, patchy grass around your drainage system. When water doesn't drain properly, it can lead to soil compaction, lack of air circulation, and other issues that prevent healthy grass growth.
Some common symptoms of drainage problems include standing water, muddy soil, and grass that looks thin, yellow, or wilted. To fix this, you may need to improve the drainage system by clearing clogs, adjusting the slope, or adding drainage pipes or a French drain. If the issue is more extensive, it's best to call in a professional landscaper to evaluate and repair the drainage.