[Question] How to iterate when we have prototyping tools and usability studies?

I've been struggling for a while trying to convince teams that we should be developing our applications with iteration instead of being purely incremental. My go-to method for explaining this is Jeff Patton's "Iterative Mona Lisa" picture. Teams normally come back with the following arguments:

1. The UI/UX group has provided mockups showing that they want the feature to be designed "this" way. These designs have already been "iterated" by using high fidelity prototyping tools like JustInMind and testing them through usability studies with the customers/clients.
2. If Product comes to us wanting a car, we know we can't build a car in one sprint. We may be able to build the frame in one sprint, but to ask us to build a skateboard instead of a car frame would just cause of more rework and delay completing the car.

The other important detail in these situations is that the organization cannot actually release at the end of every sprint (2-weeks). Everyone knows that and there's no willingness to pretend that we can deliver at the end of every sprint.

Now that I've tried painting the picture, how would everyone handle a situation like this? When the organization has a UI team that iterates using prototyping tools and performs usability studies, how does one convince a scrum team to iterate their development?

*Side Note* The UI/UX team is considered their own team. They attend scrum refinement sessions for the teams they're working with but they are not quite embedded into the scrum team.