Only because you make invalid assumptions about how it must have evolved. Lets start with an amphibian and egg. Now lets say that a mutation causes the exterior to be a bit more rubbery. Initially 10% of hatchlings that could have handled the tougher exterior can't get out, but 10% more eggs survive being trod on by large animals. Except it's not static. Each generation that gets out of the egg has a greater concentration of the genes that give them the strength to escape the tougher egg. Repeat the process a dozen times over the course of a million years. Eventually you reach an equilibrium; the shell can't get tougher because the resources needed to escape it are expensive enough that the animal would have a higher energy burn, and fare poorly in times of drought or famine.
Fast forward a few tens of thousands of years. Another mutation causes the animal to develop one tooth earlier than it should. It's weak, but it allows weaker hatchlings to escape an egg of equivalent strength. The mutation spreads, aided by the occasional drought of famine, where the "weaker" animals survive. Later, another mutation makes this early, poorly formed tooth drop off; it was getting in the way, and it's better to grow strong teeth later. The egg shell toughens more and more, and starts becoming less water permeable as some individuals find a niche laying eggs near the water line where egg eating marine life has less access to it.
Lather, rinse, repeat. Tougher and less water permeable eggs make the eggs survive more often, and in more places. Small changes can be compensated for with existing intra-species variation, but if a novel mutation arises that deals with the costs of the new strategy more effectively, selective pressure will spread it. Follow this chain of events for a hundred million years, and you got from fish to amphibian, and from amphibian to reptile. It's not a whole bunch of lucky coincidences at once, it's one coincidence, adaptation to take advantage of it, then another coincidence and further adaptation, over and over, over the course of millions upon millions of years. It took billions of years to go from single cell life to multicellular life, a hundred million years to go from marine life to amphibians and so on. This is a mind-boggling scale of time; continents circled the globe in the time it took for mammals to evolve from reptiles. You don't see the continents shifting, but it happens all the same.
The tiny changes and recombinations occurring in animals today won't produce many new species "naturally" in your lifetime, but over the next 10,000 years? Million years? 100 million years? I wouldn't bet on animal life remaining unchanged.