My personal guess: Oversimplification and effects!
There's no reasoning, they either don't want to explain details or just go for the visuals and just assume people won't notice or care. Especially in the case of computers monitors are the typical "tip of the iceberg" for most people. That's the computer for them, because it's the visible part you're interacting with.
However, it's not necessarily deliberate. Many authors just create their story outline or probably the whole script without actually knowing what they're writing about. Otherwise you couldn't explain storyline stuff such as nuclear weapons fitting into a small pocket or supercomputers in credit card format.
Unfortunately such inconsistencies or simply stupid "hacks" are something you can find related to pretty much anything that's not necessarily common knowledge and depicted in movies as well as other media (like video games).
Some further examples:
Hacking: Yeah, the classic. You just connect some cable to another computer, hit a key, and the stars popping up in the password field, unlocking the system without ever triggering any systems that would avoid brute force attacks. Not to mention it's rather often possible to "decrypt" a password (or nuclear weapons launch code or similar) digit by digit.
Hacking 2: The elitist hacker is able to hack all kind of things in the victim's vicinity, no matter whether it's connected to some network or even digital at all. Screens blow up, ovens start working, etc. There's usually no believable/explainable reason behind this other than "for the effects".
Hacking 3: (Just so many possible examples!) To hack any system, you just need some kind of network connection and sometimes not even that. Military supercomputer? Super secure and intelligent door lock? Alien mothership? No problems. The specialist's computer is always 100% compatible and immediately has the right tools to compromise the target system.
High Tech Tools: This is a bit vague and partially related to hacking. Take Knight Rider as an example. KITT is able to remotely unlock locks, even if they're purely mechanical. He's able to engage the breaks of other cars, he's able to show images of cars being hundreds of meters away, possibly even behind a hillside or wall, etc. Oh and not to forget being able to isolate and listen to pretty much any telephone line, putting the NSA to shame. Or how about needing a secure telephone line, but that mobile phone isn't secure? Just put it in the other device, let some lights flash up, and the line is secured (wtf?).
Exploding vehicles: This is pretty much similar to "killing computers". See that bad guys following the good guys? Shoot the car once or twice. Chances are good, they won't just have to stop due to some cut fuel line and fuel dripping out of the car or some destroyed tires. The car will simply explode. At the same time, the vehicles of the good guys are usually extra armored and probably drive off with something like 0.00001 gallons every 100 miles.
Or how about the extreme opposite?
- Injuries: There are many instances where either a good or bad guy is injured in some way and gets away with it, without medical treatment or anything. While this is certainly true for minor wounds, there are tons of more extreme cases (how about Harvey Dent from the Batman universe?). Some characters suffer from their wounds or might even die, but it's pretty much always part of the plot line to introduce some kind of emotional moment or handicap (or to add some reason for a character's future actions).