The answer to 'why is my mulch thin grass' is that thin or sparse mulch can cause grass to thin out and struggle. Mulch plays an important role in retaining moisture, suppressing weeds, and insulating the soil for healthy grass growth. If the mulch layer is too thin, it won't be able to effectively perform these functions, leading to thin, patchy grass.
Some common causes of thin mulch include natural decomposition over time, heavy rains or wind that displaces the mulch, or not applying enough mulch initially. The ideal depth for most mulch is 2-4 inches, but this can vary depending on the type of mulch used.