Let me turn it around, if the Z7 is the premium model at 3400 bucks you are getting a whole lot of camera for 2000 bucks with the Z6 as the only difference seems to be the sensor
It's not a "premium" model; it's simply the high resolution model.
D3 vs. D3X also differently priced, the build was the same. 5Ds R is similar to 5D III but for the sensor, but more expensive.
It's not due to features, build, difficulty of manufacture etc. but simply that the high resolution is seen to be
valuable to the user and hence the user is charged more. Most users do not need it therefore a lower price is easy to justify simply because the cost of R&D per unit is smaller.
Similarly, e.g. Panasonic and Canon charge extra for Log recording. It's a lookup table thing and costs absolutely nothing to add. It's valuable to the specialist user, but most users don't need it, and hence they ask money for it.