They need to be in order to catch their prey. The ancestors of cheetahs and the ancestors of today’s gazelle have been chasing and being chased for millions of years. Therefore natural selection has favoured them to get ever faster and faster in what we call an ‘evolutionary arms race’, where traits just get more and more extreme….it’s the same process that makes, say, male animals get giant antlers or tusks.
Cheetahs have special characteristics (adaptations) that allow them to run really fast. Their whole body is slender with hardly no fat, they have long, slim but muscular legs, a flat tail that helps balance, claws that stick out and allow them to change direction easily and very large noses that allow them get a lot of oxygen from one breath which makes running a lot easier – I’m sure you know how it feels to be out of breath and how that can slow you down!