About fifty years ago, a very important discovery was made. Using the Doppler effect, it was found that radar could be used to track airborne targets. Plugging this tracking system into a heads up display allowed fighter pilots to more easily target their enemies. Further advancements led to guided missiles and radar-guided anti-aircraft guns which could drop enemy air forces out of the sky with ease. By the end of the twentieth century, flying into enemy territory defended by anti-aircraft guns with a conventional airplane was suicide.
100 years into the future, these technologies no longer exist.
In nearly every science fiction movie or video game, ship or ground-mounted guns seem totally incapable of being aimed effectively. Fighters dodge LASERS. Photon torpedoes frequently MISS. The everything sensor that can tell you exactly how many humans there are on a planet somehow cannot effectively track targets a half-a-mile away. The very existence of starfighters in fiction is usually due to this trope as, in reality, barring the invention of new stealth technologies, lasers would never miss and the Death Star would have had more than adequate defense.
The biggest reason for this trope's existence as mentioned above is to facilitate awesome dog-fighting sequences... IN SPACE!!! In order to excite audiences, film makers and video game developers tend to fall back on the war they and the audience knows best, that is World War Two. By an odd coincidence, that also happened to be the last war before the invention of the radar sight. In fact, the average citizen is often unaware that such technologies exist (they may have heard of laser guided missiles, but radar controlled guns are a little beyond them), thus they find it perfectly believable that people will still be aiming gun turrets by hand and pulling triggers in the thirty-first century. That, plus the Rule of Cool
tends to equal Willing Suspension of Disbelief.
Of course, if the writers actually cared, they might come up with new hocus-pocus stealth or jamming technologies that would render targeting computers less effective. Cloaking devices and Minofsky particles however tend to be far less common than starfighters, probably because writers find them totally unnecessary for convincing their audiences.