The short answer is: Girls get periods because they have a uterus (womb). Guys don’t have periods because they have don’t have a uterus.
The long answer is: Girls and guys have different reproductive organs. When girls get to a certain age they go through puberty and start to menstruate (have periods). There are other changes that happen too – girls develop breasts and pubic hair. Guys don’t have periods because they don’t have a uterus, but their bodies develop and change too – the changes are just different. For example: their voice changes and they develop hair on their face and other parts of their bodies.
So, although guys don’t get periods, their bodies do go through changes too.