Why battery health is it decreasing

With EVERY full cycle charge of your battery, it's health will decrease little by little. Make sure you are charging at night, while you sleep and that you have Optimized Battery Charge turned on in Settings > Battery > Battery Health. As KiltedTim correctly suggests when the health of your battery reaches 80%, it's time to have the battery replaced.

There is no way to stop battery health decline.

Because rechargeable batteries naturally lose their ability to hold a charge with time and use. it’s normal. When the battery health reaches 80%, the battery should be replaced.