Why do muslim women wear jeans?

Because jeans aren't immodest. If you wear long shirts with jeans, then you're fine. Although I have utmost respect for my sisters who take modesty to the next level, such as, Orthodox Jewish women, or abaya-wearing Muslimahs, modesty doesn't have to mean looking old-fashioned or frumpy. Have you seen how many hijabi fashion blogs there are? Or hijabi fashion/beauty gurus on youtube?