Probably.
It essentially comes down to supply and demand. While the demand of nude shoots of these models is not supplied the demand remain high. Once they fulfill that demand, the value of other shoots is diminished.
Take Jordan Carver as an example. While she refuses to publish nude\topless shots, we keep looking for them. If she ever decides to supply that demand, the prospective value of any future shoots is diminished and to regain the value they have to move to new levels (topless to nude, nude to explicit, etc.).
It's basically a business model to keep the audience 'wanting more'.