You can argue that millennia ago, when men ejaculate, only one sperm is produced. So the probability of the sperm came contact to the egg is low, thus resulting low re-productivity in human race. Then come the men that ejaculate two sperm at a time, doubling the productivity and thus his race overtake the one sperm men. then come the three sperm men, four sperm men ...... till today, the million sperm men.

This is the so called Natural Selection in process.