The answer to 'why is my shut-off valve' is that the shut-off valve may be malfunctioning, preventing you from fully turning the water supply on or off. Common causes include mineral buildup, corrosion, or the valve becoming stuck or worn out over time. Symptoms may include a valve that is difficult to turn, a dripping faucet even when the valve is closed, or water still flowing when the valve is supposed to be shut off.
In many cases, the shut-off valve can be repaired or replaced as a DIY project if you have some basic plumbing skills. This may involve disassembling the valve, cleaning or replacing internal components, or replacing the entire valve. However, if the shut-off valve is located in a difficult-to-access area or the issue is more complex, it's best to call a professional plumber to diagnose and fix the problem.