And often the ui is conveying sophistication and has not much content. Which is the opposite of what you want in a usable ui. That's why they look so cool and nothing like our regular uis.
There are some exceptions, like in the film "her".
That is also the reason many filmmakers like to put the ui in the air (holograms) or on glass. It looks cool on camera and you can have the actor plus the ui in the same shot.
It is! And what the production designers put on screens is part of what communicates with the audience. Maybe the audience needs to get specific information from the UI in order to understand something in the story. Maybe the audience needs to understand that the talent is having a certain kind of interaction that gives them a particular kind of information, or allows them to control something. In all those cases, the UI is part of how the creative team tells the viewer what's going on in the world of the film!
Yeah the functionality is definitely important from story telling perspective, my point there is the style of the UI is also part of the world building, e.g. the screens from Star Wars should look different from the screens from Blade Runner cuz they're different worlds and different culture, people should have different means to access information from devices. Making everything typical sci-fi style hologram looking UI works but it's a place that can add some nice touch (or can use non screen interfaces if it makes sense in the world)
What you show and how you show it is conveying something to the audience.