Great question! There are a number of reasons why. This first is genetic – the genes we inherit from our parents which are the instructions on how to build our bodies. Some people are genetically predisposed (more likely) to be taller, shorter, more muscular etc. than others, because those are the genes that got passed down to them from their parents.
Another reason is environment – which also describes how people are raised. Children are at a greater risk of becoming obese if one or more of their parents is obese. Does the child have access to exercise and a healthy diet? Do they do weightlifting in their spare time? These factors are very important, and can often outweigh the influence of their genes.