tendons/ligaments do strengthen over time, they just take much longer to strengthen than muscle, or skeletal tissue. Consequently they tend to be the weak link. Think of a power lifter who can squat 600 pounds. The were not able to do that 5 years ago, but with years of training, when they started at being able to just squat 400 pounds, the muscle and ligament tissue adapted and grew to be able to withstand the stress of their surroundings.