In the software industry, not in all cases but true for many, you usually report to a manager who has no advanced knowledge about the underlying technology used in a software project.
Usually this drives managers to value your work based on what they see on the screen rather than what's really required to get the project done. You're chained to show something to them and forced to work on poor design decisions to just get an ok, keep doing.
My concern is if it's actually unrealistic to think that more involved and engaged stakeholders shouldn't value you on what they see on screen, but by the actual already achieved project goals, either if they might have an impact on the UI or not.
In fact, when projects require not so useful demos, overall cost is increased, because preparing a project to work with quality requires testing, debugging and even last hour modifications that might be thrown away once the demo is done. Finally, there's a timing debt associated with this kind of project management that impacts on you because they delay their own timings without modifying them (wasted time is lost forever).
In the other hand, I'm not arguing that we should hold them 2 years until they can see something on the UI. IMO, I would say that agile project management with approaches like Scrum already define the sprint demo, but based on my experience, stakeholders don't want to get involved in regular project management, but they just want a global feedback with a demo that should look like a final product in terms of quality.
I want to be sure that no one could understand that my question is arguing that I shouldn't demo the UI. Furthermore, I'm not against showing it very often. But as some have already said in their own answers, sometimes it takes time to see some change on the UI while some other details are being implemented, fixed or improved.
So, that's why my question is about if it's unrealistic to get reliability not just by what you see on screen. Thus, it's implicit that UI demos are required. End-user feedback is crucial to succeed on delivering a good project, because after all the project is for the user.