The DMG 1e mentions (pg. 35) that only 1% of humans will have character-type classes (among other races, the ratio is 2%). Much like the typical game has way too much magic, I get the impression that the typical game also has way too many skilled NPCs.
The distribution of classes is listed as follows:
Clerics 20% (Druids 1/6 of those)
Fighters 44% (rangers 1/10, paladins 1/10 of those)
Magic-Users 20% (Illusionists 1/6 of those)
Thieves 15% (Assassins 1/6 of those)
Thus, in a decent-sized small city of 10,000 people, we can expect a total of only 100 adventuring class NPCs:
17 magic users
Pretty low numbers, if you think about it. In a small town of 1,000 people, there would only be 10 people with adventuring classes:
2 magic users
Think about the implications of that. It would take a small town of a couple thousand people just to equip the most basic of adventuring parties. In a village of 100 people, you would expect to find only find 1 person with a character class.
Any human is capable of picking up a sword and swinging it around, but the fact is, such PC equipment is bound to be pretty rare too. I mean, really, why would the average citizen have a sword lying around? They wouldn't have the money to buy it, nor the money and skills to maintain it, and it might be illegal to own in the first place.
In reality, the average citizen is going to have only "peasant's weapons", and no armor to speak of. For self-defense purposes, the average citizen would have a wooden club to bash things with, and a sharpened stick or second-hand spear to poke things with. In a pinch, they might swing an axe or scythe. There is a small chance they would have a small shield lying about, but that is it.
Finding a henchman or a pal to adventure with is going to be difficult. Only about 10% of those with class skills will be interested in adventuring employment at a given time, the rest being satisfied with their current situation. Thus, I think we can see the logic in why most fictional adventures took place solo, or just the hero with one sidekick.
Our idea of a "well-balanced party" is more or less just the product of the power-gaming mentality, wanting to be able to effectively hack-and-slash through any dungeoneering challenge. Adventuring with only one or two PCs makes the game playing much more challenging and high stakes. Thus, even Conan resorts to sneaking around, using his head, and planning good strategies, when it is just him and a couple low-level followers.