I once heard that a kabbalistic explanation for this, Unfortunately the moderator has edited out the explanation for modesty reasons
As for other things such as shomer negiah, they are all in place to prevent men and women from giving into their temptations and doing something wrong. From the kabbalistic view I heard, the hair only becomes an issue after the night of the wedding, so it doesn’t need to be covered beforehand. Touching someone can cause issues before marriage, so it’s forbidden then also.