I am a little bit tired of how over prevalent it is, HOWEVER. I can provide to you a real and practical explanation as too WHY boiled down into a couple of points.
1) Armour, or lack thereof. see feudal japan had a bit of a problem, they didn't have that much high quality metal (compared to europe where you couldn't spit without hitting the stuff) as a result the stuff they did have they would use very carefully which resulted in a very high quality blade. It also ment that chainmail didn't really exist and full plate was beyond their scope of imagination. as such the katana was designed as a slashing weapon not a bashing or stabbing one... why is this relevant? most movies that have katana's "out of context" are in modern settings, and how often do you see someone walking around in full plate.
2) So why not a European blade then? While I do love european blades and for pure cutting power on an unarmored opponent they are about equal. so why no love for the claymore? Well, weight for one. european blades tend to be quite a bit heavier with few exceptions (that I will get too in a second) and harkening back to point one they where designed this way to crush armour. now since as mentioned these out of context sittuations will 9/10 have us against an unarmoured opponent... which would you choose? a big heavy sword or a light fast one when both will be just as effective?
3) The rapier and its cousins, now here is a group of European swords that are lighter and faster. so why are they not as popular. well here it starts to boil down to fighting aesthetics tbh, rapiers where invented in a time where having SOME armour still was a possible issue, as such they are stabbing and thrusting weapons so they could slip between the cracks etc. why is this an issue though, well it has to do with the visuals. stabbing and thrusting may be fine for actual fighting but it simply doesn't read as well on camera as the large arching swings of a katana do. not for todays big action sequences at least (See the fights of the princess bride or any of the eroll flynn movies for examples of how it CAN work really well)
4) Scimitars and other swords from the near east. "terrorists use em" unfortunately there is still a large blanket animosity towards anything even remotely arabic in alot of western nations so these swords sadly shall remain strictly in the hands of bad guys for the time being.
5) Sabres... PIRATES! This sword has been so strongly associated in the publics mind as being the "pirate sword" that you put it in the hand of ANYONE and that is what they will instantly think of them being... so unless the character IS a pirate, or at least thinks he is, your not gonna be seeing him use it.
So, to wrap things up why is the katana the most over used sword out there? well it comes down too a combination of societal pressure, cinematic aesthetics, and pure practicality.
Edited for qualifications:
18 years martial arts training in ju-juitsu and bu-juitsu
5 years of teaching in said arts
certification in instruction of european weapons
4 years film industry experience.