Why is it that frog eggs have to be laid in water?

Frogs must lay their eggs in water because otherwise the eggs, known as frogspawn, will dehydrate and die.

However, not all species of frogs lay their eggs in water.

those that don't include the Corroboree frog, as well as frogs of the genus Pristimantis, which lay their eggs on land, where they do not undergo a tadpole stage, hatching as fully metamorphosed baby frogs. Some species of frogs even give birth to live young, such as members of the African genus Nectophrynoides and other species found in the Andes and Central America.