Because diabetic nephropathy is a silent disease in the early stages.
You may have no symptoms at all for many years while the kidneys are slowly being damaged. Most people only discover they have kidney disease when it is already advanced, and the kidneys have lost a significant amount of function.
By the time symptoms appear—such as swelling in the feet and ankles, tiredness, poor appetite, or decreased urine output—the damage is often irreversible.