I think you lack imagination. It all comes down to cost and convenience. Mounting something on the exterior is going to be a specialized process whereas getting new windows is relatively standardized. Then add to integration with the electrical system. Theoretically this could be like balcony solar. An electric plug under a window is not uncommon. So if the cost for a solar window is not that much more than a standard window to some degree. It would make sense. Its not a crazy thing to look into at all. Now granted might as well make the exterior frame collect solar as well if possible but again that might just end up being to expensive relative to surface area. Would be worth looking into though just like the window.