Why must most amphibians start their life in water

Because it is what amphibians are adapted to live in. They are adapted to wetlands. it is where they eat, mate, sleep, etc ...

Frogs and toads live on land but require water in which to lay their eggs ... Frogs live on both land and in water and require habitats near ponds, swamps or other damp places as death occurs if their skins dry out. Frogs have permeable skins allowing them to both breathe and drink through their skins.