That’s a great question. Aristotle wondered about this in Ancient Greece. Modern biologists usually try to answer questions like this by looking at the costs and benefits.

Living things don’t get mutations because they need them. Mutations are random. But once the genes exist for something, they can give an individual an advantage, making the genes likely to become more and more common in future generation. Or they can be neutral, and not be any more or less likely to spread. Or they can be a disadvantage, and be likely to disappear.

Having a tail that breaks off easily helps lizards escape from predators. That would give lizards who had the ability to re-grow tails a big advantage, so they would be likely to leave more offspring with genes that help to re-grow tails.

With other species, like a cow, losing a tail might be so rare that an individual with the tail re-growth genes may have no advantage, and the genes never spread. Or losing a tail may be so terrible that individuals don’t live long enough to re-grow the tail, which would also mean the genes would have no benefit.

Are there any costs to re-growing tails? Well, a new tail is a big investment of energy and nutrients. For lizards, the investment is worth it. The tail may save their lives another day. For other animals, it might not be. There might also be some really complicated reasons that have to do with the control of cell division and how cells change to become different types of cells.

Think about the costs and benefits listed above. Why do you think humans can’t re-grow a lost arm or leg?

You might be interested in studying developmental biology or animal physiology.