Because it is not necessary for the game.
By that logic, the game should've been released with just swords and a single suit of armor and modders could've added the rest. More than one face per gender per race? Let the modders add them. I don't mean to offend, but I have to say that I find your reasoning to be infuriatingly terrible.
The only difference would be the model, animations, and whatever values you want to assign them.
See above. Axes and maces add nothing that swords don't, except for some resemblance of sanity. If you want to make a world that seems alive, you better not [censored] everything up by making it a living bizarro world. Spears and clubs are the most basic weapons imaginable, and their uses are many in a world without firearms, so not having them around is a brain sore unless the angle of the game is very narrow. Needless to say, an open world game with plenty of guards, a regular army, loads of horses, dragons, bigass bears and cats, and the odd troll is hardly the place you'd expect to not find a single [censored] spear or polearm.
If that doesn't scream bizarro world to you then I'm sorry to say this, and I mean it in the least offensive way possible, but then you would buy any excuse for a world, no matter how garbled and senseless it is. Nobody in their right mind would get into any kind of swordfight with an angry bear and given the amount of bears in Skyrim, the lack of pole weapons is beyond bizarre. gamesas are essentially asking players to accept that just about every single damn NPC in Skyrim is either bat[censored] insane or massively suicidal.
And please, don't give me nonsense about how Tamriel isn't medieval Earth. It doesn't change a damn thing, as long as bears are bears and humans are terribly vulnerable to high force impacts from sharp or blunt objects, be those bear clows, giant clubs, mammoth tusks, or a sabercat tackle + bite + claws combo.
It does not make sense for them to go to that trouble.
This is true. It does not make sense for them, from an economic point of view. A sensible, deep game world that makes the least bit of sense is a quality they don't need to sell millions of copies of the game, because the average casual gamer wouldn't notice even the most glaring inconsistencies in the world. Consequently, it's good economic sense not to waste resources on something that isn't important for achieving success.
Economic sense, however, is not something that anyone not hired at gamesas has to give two dog turds about. We're customers and as customers, it is our job to make realism critical for success, assuming we don't want some Ducktown realm of crack-inspired nonsense.
Here is a simple question, why outside the fact do you think that Spears and Polearms should be included and what Perks would you associate with them - further how would this be any different than Axes, Maces, or Swords?
Heh, what perks? As if the sword or axe perks are even remotely significant? The blunt perks add armor penetration, which is ridiculous since the armor penetration is a question of physics and should happen regardless of perks. In that line of reasoning, you could've given spears a stagger effect or an "extra damage vs piercing-vulnerable targets" or maybe just something as "lame" as a damage bonus vs animals? It's not like any of the specialization perks aren't more or less useless anyway, so why does it matter?
As for why spears are different from other weapons, well... I, for one, think poking stuff with a long stick is HUGELY different from stupidly walking right up to it and waving some damn knife around in its face. That's just me, though. I mean, what the [censored] kind of question is this? Either the world makes a tiny bit of sense or it does not. If you have mounts, a Roman themed civilization, and a lot of large game to whom even large men are really just squishies, then a solid supply of long-range weapons to keep the "squishies" of out "squish"-range is a hundred times more necessary then half a dozen kinds of [censored] sugar treats or what have you.
It's a simple question of basic survival, something your garden variety Nord certainly wouldn't do for long if he was to take on trolls, mammoths, giants, bears and sabercats at sword range on a regular basis. You might as well take out fire or food or freaking houses, because we're talking about a tool just as essential for survival.