Why is energy from the wind renewable?

Wind like waves and sunlight are created by natural functions of the earth as a planet. They do not require time to form a fuel like coal and gas. As these functions are "naturally occurring" they are considered to be renewable. They will, as far as we are currently concern always going to occur.
There is always wind to use to generate energy. The wind is not going to run out. Energy sources like coal and oil will one day run out