Why do other moms I see like my friends and women at swim lessons not have stretch marks but mine are dark and there are a lot of them?

Genetics. Stretch marks vary from woman to woman during pregnancy and are based on factors such as excess weight gain and genetics. It is all normal.
Good question! Some people get stretch marks, others do not, and the reason for that is not yet known. There is some evidence that stretch marks have more to do with hormonal fluctuations than with physical stretching, which would explain why they are most common in teenagers, pregnant women, and obese people (fat cells secrete hormones). But it still doesn't explain why some get them and others do not. Mystery!