It was done as an art form, or physical exercise, or in attempts to be dead. There's no reason you would want to have a second sword over a shield, especially if you're using swords light and small enough to use easily in a single hand. Most medieval European fighters either used a sword and shield, a hand-and-a-half/bastard which was meant for use without a shield and used in two and at times one hand, and longswords, which were pretty strictly two-handed. You could say greatswords were also commonly used, but they were more so ceremonial, with only the biggest, brawniest dudes using them--though they were definitely still used, namely by the nobility that grew up well-fed and nurtured, making them grow taller and larger than most average people. To actively strive to fight better with two swords is something an idiot like Aerion Targaryen would do. And if you're so good a fighter that you're given your house's ancestral sword that was made out of a fucking meteor and only ever gets bestowed upon the house's absolute best fighters throughout history and is otherwise locked up in a vault, you're going to practice how to fight well with it. And if it's a greatsword, you practice and fight with greatswords.
Spoiler
And if Dawn is Lightbringer like some theories say, it's stupid for them to just ignore it. The fight in the books was set up so Arthur Dayne had this great fuckin' greatsword that's nearly magic (possibly Valyrian since they take a lot of similar traits as the Valyrians), and his two other kingsguard pals, Oswell Whent (who funnily enough is portrayed with two swords in his wiki picture), and the Lord Commander Gerold Hightower, who was likely a better fighter in his old age than Jaime ever was. Why amalgamate the two? It's seriously just to make Arthur seem badass, I feel, and it really gives off the wrong message. The show shouldn't be about eye-candy, but it quickly has become just that.
If all fight scenes were realistic then they'd be over VERY quickly and would also be boring to watch, which nobody wants.
Not necessarily. A fight like this and its outcome is pretty unique in that the outnumbered party (7v3) managed to kill all but two of the other. That implies a lot of readiness, quick thinking, tension, suspense, etc. etc. You can't tell me you've never been in a Warband match where the game falls down to a handful of people, and though the fight may last a while, it's never boring, and in fact, quite engaging and interesting. That's a game with 2-4 attack and block directions. But nah, two swords flipping around idiotically while guys twirl around and hide behind shields is waaay better. That entire fight scene went by and I was just sitting there like "wat is hapenin."
