Why do visible structures such as fins, beaks, and tails, look so different on different organisms?

They've been specialized so they best help that organism survive, and is affected by conditions like environment, climate, food, etc. For example, one fish may have particularly shaped fins because it needs it to survive in the strong currents where it lives, but another fish may not need such fins because it lives in shallows. Things like that have affected the appearance of things like fins, beaks, and tails.
Hope that helped you.