Why do fish and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?

Animals like frogs and fish lay hundreds of eggs and release millions of sperms. But the entire eggs do not get fertilized and develop into new individuals. This is because the eggs and sperms get exposed to water movement, wind and rainfall. Also, there are other animals in the pond which may feed on eggs. Thus, production of large number of eggs and sperms is necessary to ensure fertilization of at least a few of them.