The answer to 'why is my sod overgrown' is that your new sod is likely growing too quickly due to ideal growing conditions or over-fertilization. This can cause the grass to become thick, lush, and unmanageable. Symptoms include the sod growing much taller than your desired lawn height and becoming difficult to mow.
The best solution is to mow the sod more frequently, at the proper lawn height, and avoid over-fertilizing. If the growth is still excessive, you may need to aerate the soil and apply a low-nitrogen fertilizer to slow the growth. However, if the issue persists, it's best to consult a professional landscaper who can assess the specific conditions and provide guidance.