Because there are two sorts of random numbers that you may need from a system. General random
numbers, which don't need to be truly random, but are fine for various applications such as
statistical modeling, games, etc, and cryptographically random numbers, which really truly
need to be as random as possible. You need entropy to generate the latter, and the more
numbers you generate, the more clues there are to work out what inputs the algorithm had, so
you don't want to just use the latter for everything.