Everything needs a source of food. Only plants can get their food directly from the ground. All other living things have to eat plants or other animals.
The world is very varied and this gives a chance for many kinds of plant to find their own environment to live in. This, in turn, gives a chance for an even greater variety of plant-eating animal to find food. And, in turn, this gives a chance for many meat-eating animals to find their food. But there must always be more plant than animals can eat, and in turn, there must be more plant-eaters than meat-eaters on Earth.
