why do whales and dolphins live in the oceans and not in water? we all know all mammals need to breath but whales and dolphins why cant they live on land?

The whales and dolphins are very big in size, so they need large area for their movement and they need a considerable amount of food. Freshwater lakes couldn't support a whale's nutritional needs. 

Whales and dolphins breathe air through their lungs like other mammals and therefore must surface regularly. The whales nostrils are located at the top of their heads and are called 'blowholes'. Whales come to the surface or jump into the air regularly to inhale and exhale air through these blowholes.  

 There are several reasons that why whales and dolphins cannot live on land and these are:

1. They have very large bodies that weigh many tons and their bodies are not designed to hold that kind of weight on land.  

2. They have a thick layer of blubber (fatty tissue) and due to lack of water the whale would quickly overheat and dry out. So cold temperature is important for their survival.

3. They lack appendages that would allow it to move on land and would not be able to hunt for food.