Hmmm......Perhaps spears are un-used in Mirrowind because of the land area? It is a mountainous region, with many swamps and pools of water. Maybe they don't use spears merely because it's to difficult retrieve them, if thrown, attack with them, if climbing up a mountain, and other such things. Or perhaps they have advanced enough that they no longer need the spear, as you said it is the "most basic of weapons". The second would by Cyrodill's only real excuse, since there are many hilly areas, and even in the mountains there would be flat areas.
As for the crossbow, perhaps it is lost. Maybe Cyrodill lost touch with that weapon, as did Morrowind. Though it's hard to believe that a place filled to the brim with Dwemer remains has not one crossbow....or maybe there was. Maybe it an unrecognized weapon that an adventurer stumbled upon, then sent back to the mainland, or to a Dwemer Reshearcher, to discover what it is and how it works. And our charactor starts off just a few weeks after this was found, meaning it wasn't in mass production- nor any production- yet.
Your theories fall apart when you consider that Morrowind DOES have crossbows, it's Oblivion that doesn't, and the reason for it, in this case, is indeed purely a matter of game design. There is no lore explanation for it. Even if one assumes that crossbows were some form of lost technology, which seems unlikely, given that they weren't exactly rare in Morrowind, even if they did only go up to Dwemer quality, I find it unlikely that there is any justification for spears not existing in Cyrodiil.
Impulse wants me to say "it was just game mechanics," but then I remember that if mods can make it happen (and they have), it couldn't have been an impossibility. I am now thinking that it might be one of the products of Bethesda's streamlining of the game. For example:
-Gauntlets now are a single unit instead of two.
-Pauldrons are fused to curiasses.
-No long/short blade differentiation.
-No medium armor.
Obviously it's NOT that it couldn't have been done, most likely, Bethesda simply wanted to, as you say, streamline the game, or just got lazy and didn't want to bother with another type of weapon.
I would say archery in Tamriel is not that well developed, because anybody can fire any bow. It is not even possible for you or me to pick up and fire a 100 pound bow without considerable training. Archery in Tamriel seems to be more of a thing for hunters, who happen to use their bows for fighting on occasion. If some culture were to develop stronger archery, then they would e unstoppable for a while (because Tamriel has apparently never seen this before) An arrow fired by a 200 pound bow can do much more damage to your body than a bullet, think about that.
Anyone can also fight with a claymore in Tamriel, or really any weapon. While your effectiveness with a weapon is determined by your skill, the game does not have any skill or attribute based restrictions on what sort of weapon you can use. So anyone being able to pick up a bow and use it may simply be a matter of gameplay mechanics, alternately, there could be http://tvtropes.org/pmwiki/pmwiki.php/Main/DidNotDoTheResearch.