Why are whales not grouped in fishes?

Whales belong to class Mammalia while fishes are included in class Pisces. Both are aquatic animals and chordates. However, both differ in several basic characters, such as: (l) Whales are warm-blooded animals while fishes are cold-blooded. (ii) Whales respire through lungs while fishes respire through gills. (iii) whales possess mammary glands which are absent in fishes. (iv) Whales have four chambered heart, while fishes have a two-chambered heart.