The answer to 'why is my shut-off valve' is that the shut-off valve may be stuck or not functioning properly. This can be caused by a variety of issues, such as mineral buildup, corrosion, or physical damage to the valve. Symptoms may include the valve being difficult to turn or not completely shutting off the water flow.
If the shut-off valve is not working, it's important to try to diagnose and fix the issue as soon as possible. This may involve trying to manually turn the valve or using pliers or a wrench to loosen it. If the valve is severely stuck or damaged, it may need to be replaced. In some cases, the issue may be with the plumbing system itself, such as a blockage or leak, which would require more extensive repairs.