Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Explanation

In some oviparous animals, like frogs, fishes, the zygote develops in the external medium and are put to predation of harsh environment. But in birds, reptiles, the zygote is surrounded by a calcareous shell and is laid in a safe place in the environment for protection.
In viviparous animals like mammals, the zygote develops inside the body of the mother where it receives proper care and protection. So the offspring of oviparous animals are at greater risk of survival as compared to offspring of viviparous animals.