Because animating something to burning to death is kind of horrifying. I’m sure they could make them instantly poof into ash or something, buuuut…still a kid’s show.
Here’s a better question: If Spike is a fire-breathing Dragon and the Timberwolves are made out of bits of wood, why is Spike running away from the Timberwolves?
Twilight is High Intelligence, Low Wisdom. Severely low Wisdom. I actually played with a guy with that kind of Wizard. He was really good at staying in character. The campaign ended up being completely insane, but it was a blast.