Varför använder de ibland inte ett riktigt operativsystem i filmer?

It could be for dramatization, or it may be caused by legal and/or financial issues. Some of the reasons are listed:

• OSs and software interfaces are considered intellectual property, and are therefore copyright protected. Most of them are or also represent a brand. It would require a paid or unpaid agreement by the copyright holder or brand owner to depict them. In 24 (TV series) for instance, CTU has been using Apple Displays most of the time while the software shown was proprietary. Apple remained uncredited. In later seasons however, 24 made heavy use of "Cisco Teleconference Systems", and showed their logo frequently. The credits revealed that there was financial support by Cisco accordingly.
• Product placement is still not legal in many countries without a "proper warning" (e.g. in Germany).
• Most OSs or familiarly known software apps don't do as quickly and easily what a screenplay requires them to do. They also don't look as spectacular and certainly not lucid enough to display the necessary clarity and/or impressiveness for a quick cut to their interface in order to reveal at a glimpse what the machine is currently chewing on.
• There is an image problem. It has been a strong habit for the last decade to show the good guys using Apple Macs vs. the bad guys using Windows PCs. It's certainly not a problem that the bad guys use Windows, but it is a problem if the good guys win, because the bad guys used Windows (hacked their system or something fictional that wouldn't -- or actually would! -- work in reality). If you look closely, you will often find Apple computers with the logo rendered unrecognizable, while the case design is obviously non-PC. (source)

Och ibland visar inte filmer / TV-program den verkliga logotypen eller operativsystemet bara för att dölja den verkliga produkten eftersom de inte får betalt för den. Jag menar att producenten / regissören inte är villig att göra gratis annonsering.

For more details on fake brand endorsing in movies and TV shows, refer to the following questions:

1) Are fictional trademarks or brand names used in movies and TV legally held?

2) Why do so many movies & TV shows feature recognizable, un-branded or re-branded products?

Kan det här svaret hjälpa dig på något sätt.