There is also the issue of the design generation - the STM lens is much newer, and there has been more experience in computer-aided design.
The 18mm of an APS-C lens corresponds to the view of a 27mm lens on full frame. 28 mm FF lenses are pretty easy to design. 16mm FF lenses are more difficult to design. Full frame zooms incorporating 16mm are even more difficult to design, particularly those that retain filter rings. Remember that to the designer there is no such thing as a "perfect" lens, just a lens with optimal compromises on price, weight, center sharpness, corner sharpness, aperture, bokeh, coma, chromatic aberrations, flare resistance, etc.
For the time being, I have gone the prime route for full frame, with a current line up of Samyang 14mm f/2.8 manual focus lens, Zeiss 21mm f/2.8 manual focus lens (a dream lens, sharp at f/2.8 out to the corners, great color and microcontrast, I was a lucky dog to find a used copy at the time I was ready to shell out for the 6D plus lens), Sigma 35mm f/1.4 , and some 40 year old legacy lenses in the 50-60mm range and 105-135mm range, my favorites being the Nikkor 50mm f/1.2 AIS and Mamiya-Sekor 60mm f/2.8 macro and Nikkor 105mm f/2.5 AIS, with adapters of course. The M42 lenses were mine from my film days, the Nikkors were from my father's film days. It is a reasonable landscape kit, but the manual focus manual aperture lenses are a PITA for action.