I suspect it's because relatively few of them actually make it to maturity. They get picked off by birds before they reach the water, then eaten by other predators once they're in the water, or they get trapped in the surf and die, etc. Having so many young ensures that some offspring -- usually the fastest, strongest, healthiest -- will survive. Natural selection. Same with frogs. Polliwogs are easy pickins for fish, crawfish, water spiders and other insects that rely on them as a food source.