The answer to 'why is my drainage system overgrown' is that your drainage system has become clogged with vegetation and debris. Over time, leaves, grass, and other organic matter can accumulate in the drainage pipes, grates, and channels, preventing proper water flow and causing the system to become overgrown with plants and weeds.
This is a common issue that can happen with drainage systems, especially in areas with heavy rainfall or in yards with a lot of trees and vegetation. The overgrowth can lead to further blockages, standing water, and even flooding if left unchecked.