common, why don't we see more evidence for them?” Why do we not see them in period Art?
Well, look back at my Feb 10, 2008 posting. There are photos of me knitting with a knitting sheath and you cannot see the knitting sheath. You just have to recognize that the knitting sheath is there from the position of the hands. (And hands get moved while posing for a a painting.) A historian that is not thinking in terms of knitting sheaths is not likely to recognize that position of the hands as signifying the use of a knitting sheath.
This would be a strange view for an artist to paint and yet is the the view that the artist would have to paint in order to show the classic use of a knitting sheath.
The artists knew their business, and painted people's faces rather than their hips.