Here's why:
* Mammals are warm-blooded animals that have hair or fur, give birth to live young, and nurse their offspring with milk. They breathe air through lungs and have a four-chambered heart.
* Amphibians are cold-blooded animals that typically have smooth, moist skin and spend part of their life cycle in water and part on land. They breathe through their skin and have a three-chambered heart.
These are two distinct classes of animals with very different biological characteristics. It's like asking if a bird can also be a fish.