The weather changes because it does.
Weather is the state of the atmosphere. But the state of the atmosphere cannot always be the same. Every minute, every hour and day, changes occur in the atmosphere: The accumulation of moisture, the movement of air masses, as the Earth rotates around the Sun, the amount of solar heat in one or another part of the globe changes. The United States has the highest temperatures in the south and the lowest in the north. In some states at the same latitude, the temperatures are very different. I follow the weather changes on this site https://world-weather.info/forecast/usa/san_diego/