Because essentially a exponent is an abbreviation for a number being multiplied by itself multiple times. For example, when you have x^3 is x*x*x. So when you have x^3*x^5, you are multiplying x*x*x by x*x*x*x*x, making it x*x*x*x*x*x*x*x which is the same as x^8.

Hope that makes sense and helped you out.

Hope that makes sense and helped you out.