Its mainly a backward shallow understanding of religion.
Go to Somalia and you will see little 3 year olds in full Jilaabib and they think its religion, it's ceeb for her to exercise or play, and this comes at the most critical time when muscles develop, the earlier you start the better.
Even here in the West I have seen them stopping young girls far away from the age of puberty from swimming classes because it's mixed, and they think this is religion.
We have a pandemic of Somali girls with knocked knees and this also reflects into the type of babies they give birth too, Indian looking, malnourished, weak.
Its a combination of a false understanding of Islam and shallow ignorance about life and health.