Why are there so many animals that look similar to each other?

why are there so many animals that very much like one another? like green tree pythons and emerald tree boas look-a-like, jaguars and leopards look very similar, crocodiles and alligators also look-a-like, some wild cats look just like domestic cats, rabbits and hares also look similar, so do rats and mice, and guinea pigs also look kind of like hamsters but larger, and why do mammals and marsupials look-a-like as well, even reptiles, amphibians, and dinosaurs have a lot in common, so why is that some of these animals look very much alike and why does each and every individual human-being besides twins and triplets look different from each other than other species like why do all tigers, wolves, some dog breeds look-a-like where as humans look different from their own species, male and females lions are like one of the only animals that you could tell apart but if you picture a male lion without a mane, he'll look like a female lion right? so could somebody explain to me why do some animals look-a-like and why do "us" human-beings look different from each other?