They're common because humans tend to get them and we are all human. It happens due to growth spurts, weight (fat or muscle) gain, pregnancy- anything that causes your skin to literally stretch. When the skin stretches relatively quickly, it doesn't have time to grow normally to make up the extra space, so it produces scar tissue.
I HATE THEM ...im 19 and I have been skinny all my life I weigh 115lbs and I have them my BUTT ... people say they can't see them when I'm in a bikini but I can. I'm so athletic too I go jogging every DAMN DAY but they are their. I really don't know why they are common.