The answer to 'why is my grass thin grass' is that there are several common causes for thin, sparse grass. This can be due to poor soil conditions, lack of nutrients, drought, excessive foot traffic, or pest infestations. Symptoms include patchy, discolored, or slow-growing grass that doesn't fill in evenly.
To fix thin grass, you'll need to address the underlying issue. This may involve aerating the soil, overseeding, applying fertilizer, improving drainage, or treating for pests or diseases. Depending on the severity, this may be a DIY project or require the help of a professional landscaper.