It's historical inaccuracy.
There are two origins for this. First, as you note, surviving old swords are often pitted and tarnished (sometimes even if well under a century old, if they are outdoor or buried finds). Second, hand-forged tools are often left with the forge scale on them, without being polished, except for the edges of bladed tools. Since there is a modern perception that "hand-forged = primitive", supposedly "primitive" weapons are often depicted with such an unpolished forge-scale finish.
Some examples of such unpolished finishes, with forge scale:
Billhook, forged in 1918: https://commons.wikimedia.org/wiki/File:Devon_Billhook.jpg
African spear: https://commons.wikimedia.org/wiki/File:Brooklyn_Museum_22.303_Spear_%282%29.jpg
Japanese kitchen knives, modern: https://commons.wikimedia.org/wiki/File:Makiribocho.Osaka.Tokyo.jpg
The forge scale is left on for two possible reasons: it's cheaper and easier to leave the tool/weapon like that without grinding and polishing the whole surface (less labour and less waste of materials), and it provides resistance to rust. In some places/times where tools were typically left with forge scale, weapons were often finely polished. The tools were not left with forge scale due to lack of knowledge or skills, but for economy and/or rust resistance.
Weapons such as swords and spears from sub-Saharan Africa were often left with forge scale. Outside Africa, a polished finish was more common for iron/steel weapons. While not many Medieval and older weapons have survived with the original surface intact, there are enough with partially-intact original surfaces to show that polishing was used. For example,
In addition, there are many literary references to polished swords:
Ezekiel 21:9-10: I have sharpened my sword to slaughter you; it is shiny and will flash like lightning!
Beowulf 213b-15a: Men bore into the bosom of the ship bright treasures, splendid war-gear.
Beowulf 581b-86a: I have never at all heard such contests, such peril of swords related about thee. Never yet did Breca at the battle-play nor either of you, perform so bold a deed with shining swords.
The Letters of Cassiodorus, 5.1 King Theodoric to the king of the Warni: "So resplendent is their polished clarity that they reflect with faithful directness the faces of those who look upon them" and "The metal is ground down by your grindstone and vigorously burnished by your shining dust [polishing compound] until its steel light becomes a mirror for men; this dust is granted you by the natural bounty of your land, so that its possession may bestow singular renown upon you. Such swords by their beauty might be deemed the work of Vulcan"
Mabinogion, Culhwch and Olwyn: "For some time I have been seeking one who should polish my sword, but I found him not."
Little Sermons on Sin: "gloves trimmed with marten which she uses to polish her face, with that toilet [i.e, washing] water of hers, until it shines like a sword", "herself loaded with civet, even to her eyebrows, and shining like a sword!"
Further, where blades were made with decorative pattern-welding or deliberately patterned crucible steels, the patterns were not visible without polishing (and usually etching and/or staining).
For bronze weapons, the same applies: usually they were polished. One spectacular find of a Chinese bronze sword with the original finish almost-intact is the Sword of Goujian.
In total, we have enough evidence to show that swords of the past were typically not pitted, corroded, and tarnished during their working life. In same cases, swords were polished to a mirror finish. In other cases, they were polished, but to a less-fine finish (e.g., polished to a satin finish). In other cases, swords had other types of decorative finishes, such as etching or stains to enhance the contrast the pattern in pattern-welded blades. One example of a late Medieval sword with ornate decoration is this blued and gilded sword of Holy Roman Emperor Maximilian I, from 1496: http://i.imgur.com/Rt0oGeG.jpg
