Of course they could add a feature that lets you simply step the resistance up or down but that’s not what virtual shifting does in Zwift. They chose to implement something more complex. This is the best explanation that I have seen so far (see below). You pick any physical gear you want, Zwift figures out how to place you in the middle of the virtual gear range, and then they replicate the resistance changes based on a specific set of gear ranges so it’s more like the gearing differences on a physical cassette. I imagine this is not satisfactory to people left behind with trainers that don’t support it.