Just as a pure speculation (I am not specialized in this area) I find it a little bit odd that forearm is not enough as I find it is the most common area exposed to sun by people at least in the last 100.000 years.

So I am wondering how come in this case selection did not chose those whose forearms can produce a lot of Vitamin D as it was probably the most exposed part to the sun.

Don't forget that many people used to bathe outside for many millenia. But I too would be interested in how clothing through generations have put selective pressures on humans, if any.