Fake UI

Justin Frankel from Winamp released a tool for fake movie sounds for Windows way back when, it's call Nullsoft Beep. Quite funny!

http://www.1014.org/code/nullsoft/nbeep/

An implicit rule in the cinema is also to beautify everything.

Also big and clear enough to be seen on screen.

Typically if you do a closeup of a computer screen in a movie/TV there's a specific action the user is performing or message they've received that is important to the plot. You want that message to be big and obvious enough that in the 1.5 seconds of footage of the screen, it's obvious to an engaged watcher what is being done. Real UIs rarely make one thing so much huger than everything else on the screen that it will immediately draw attention as fast as the film needs you to, so they make up their own.

Also if Apple or Microsoft didn't pay a product placement ad fee, we're not gonna give this space away for free...

Alternative theory: They approached everyone from Apple to Microsoft, Lenovo, the EFF, and every Elbonian penis pill spammer, and nobody would pay for product placement, so they _had_ to invent their own UI...