They don't. Some who practice polygamy for religious purposes (such as the FLDS) wear long dresses because that is the modesty standard set by their church. Others belonging to different religions or with no religion might have other clothing and modesty standards. Check out the "Related Links" below for examples.
i do not know why they wear long sleeved dresses
Yes they do. T hey wear long white dresses.
Long Dresses
Short. You usually wear long dresses to prom instead.
Girls in Kenya wear long dresses or robes.
There are polygamists of nearly every religious and cultural background around the world. Most of them do not wear clothing to bathe or shower.
long dresses
Women wear long dresses with corsets underneath the dresses. They wear comfortable boots. Maybe some jewellery and a nice hat.
they wear long dresses that cover the ankles
maybe dresses and long skirts
ladies wore long tall dresses
i believe they wore long dresses.