It might be bottled water, but its not something completely sterile and pure, even medical grade stuff (serum and the like) have expiration dates
Esentially you can't have 100% pure anything, there may be a few bacteria left that are completely harmless because their % in the water is really small, but since the bottle is closed eventually they might reproduce and end up being harmful
Specially because most bottles are transparent and the light that goes through allows energy to get through the bottle, I supose there might be some kind of liquen in there that might use the light to grow