The answer to 'why is my roof worn out' is that roofs naturally deteriorate over time due to exposure to the elements. Common causes of roof wear and tear include sun damage, water damage, hail, wind, and general aging of roofing materials. Symptoms of a worn out roof can include missing or cracked shingles, leaks, sagging, and discoloration.
The lifespan of a roof depends on the type of roofing material, but most asphalt shingle roofs last 15-20 years before needing replacement. If your roof is showing significant signs of wear, it's best to have a professional roofer inspect it and provide recommendations. While some minor repairs may be possible as a DIY project, major roof work should always be handled by a licensed and insured roofing contractor.