So I've seen teleconverters that go onto telephoto lens to increase the perceived focal length of the lens. This allows a lens to focus at one or more additional focal lengths with a cost that it might not be as sharp as a lens dedicated to that focal length.
I've seen, I'd assume, crappy wide angle converters sold on eBay. Is there a reason why first party manufacturers, such as Canon, Nikon and Sony, aren't making them for their wide angle lens? Given how people do buy teleconverters when they need a reach, I can see a market of people, repurposing their wide angle lens to get even more in the shot, at a reduced sharpness I'd assume.
EDIT: Sony does make them for their 16/20mm APS-C pancakes as well as their 28mm full frame prime but I don't see them used a lot??