It’s not only lightning that wears out surge protectors. The electricity from the electric company comes into your building in an unrefined way. The voltage is constantly, slightly, surging and sagging. Not to extremes but enough to slowly eat away at your surge protection by simply being plugged into the wall. Most electronics are not affected by this but computers can be sensitive to this uneven power delivery.
A surge protector is a power strip with surge protection. When the voltage rises above a certain level, the surge protector suppresses the excess voltage to prevent it from causing harm. Internal components called Metal Oxide Varistors (MOVs) absorb the excess voltage and divert it safely to the ground wire, preventing it from reaching the connected equipment.
Think of the surge protection, MOVs, like a cookie. The normal slightly surging and slightly sagging electricity from the power company slowly eats away at the cookie. A lightning strike may eat the cookie all at once. When the cookie is gone, so is the protection. At that point, the surge protector is now just a power strip. When that limit is reached, the surge protector will no longer protect the equipment that is plugged into it, even though the outlets may be live and still supplying power.