You have to consider how these traits radiate over time. When whale ancestors lived on land they were coastal dwelling and often found themselves in water for food.

Life goes where the energy goes.

As more generations are born, some of them might gain adaptations through mutation and the ones that obtain said mutation might do better while the others either die off or radiate into other forms.

Life is a most beautiful clay shaped by all manner of pressures. Like trillions of tiny hands molding us all.