Why is the number of offspring produced by fishes and frogs higher it is in mammals?

Frogs and fish have extremely high mortality rates which requires the production of large numbers of eggs so that a few survive. Their eggs and offspring are generally unprotected, and rely on instinct for much or their early survival. Mammals produce small numbers of offspring that develop in utero, and are cared for, protected and trained for an extended period of time before they are required to fend for themselves.