The answer to 'why is my shingles worn out' is that shingles typically wear out over time due to exposure to the elements. Common causes include sun damage, weather exposure, and natural aging of the materials. Symptoms of worn shingles include curling, cracking, missing granules, and leaks.
To address worn shingles, the best solution is usually to have a professional roofer inspect the roof and determine if it's time for a full roof replacement. Attempting DIY repairs on a heavily worn roof can be risky and may not provide a long-term solution. A professional roofer can assess the overall condition of the roof and recommend the best course of action, whether that's targeted repairs or a complete roof replacement.