To be fair, there's not (that) much worry over which armor to use, it's more a matter of knowing what is and isn't enchanted and to what degree. There's a pretty much set kit for each class that you want to work towards.
My neutral human wizards always have either Frostbane or Grayswandir (Magicbane as backup/for MR), robes, silver dragon scale mail, boots of speed, enchanted T-shirt, gloves of dex, etc.
Frosty and Swandy are a waste for a wizard, especially because you have to #offer mountains of sacrifices to get the skills unlocked. Mbane enchanted is enough of a weapon on its own along with attack spells. And you can get dagger skill up to expert easily.
Those swords barely do any more damage than Mbane even in the hands of combat classes.
I never seem to have that much trouble unlocking them if I've gotten that far.
• Many options are available:
- Many different actions available at any given time (move, attack with a weapon, Zap using a wand, Quaff a potion, Wear a different piece of armor or equipment, Wield a different weapon, Read a scroll, Spellcasting, etc)
- Many different options available within each action (place to move, spot to place a fireball or lightning bolt, place to teleport to (!!), which piece of armor or equipment to use)
As a very rough estimate from someone who hasn't played NetHack much, I'd guess that a mid-level character might have 10^4 or so options immediately available to them at any given time, though many of these are combinatorial results of having thirty different spells that can be targeted on any of the hundred or so visible tiles. The branching factor for Chess is about 10^1.5, and Go is around 10^2.
• Options are difficult to evaluate:
- On an immediate mechanical level: If you want to use teleport, what's the best place to teleport to? If you want to cast Fireball, how do you use it as effectively as possible? If you want to shoot into a group of enemies, where do you aim?
- On a tactical level: Would this monster be more effectively dealt with by spending some energy to cast Fireball, using a charge from your Wand of Magic Missiles, or attacking it in melee and risking taking some damage?
- On a strategic level: Would it be more useful to clean out the rest of this level or immediately go down the stairs? What set of armor and weapons would result in the best chances in melee against the enemies I expect to encounter in the near future? Which of my items should I cast Polymorph on to maximize the chance of receiving a random item that would be useful to me?
- On a mathematical level, NetHack suffers significantly from the horizon problem. That is, many immediate tactical or mechanical decisions have strategic consequences that require an AI to simultaneously consider all of the above problems. You can't just have a mechanical planner, a tactical planner, and a strategic planner; you have to have a unified AI that can realize that it'd be a good idea to not kill that one enemy in the corner because it can kill it and eat it two hours later on its way back up to the surface.
This is an even nastier issue. Even humans can't do this effectively for NetHack. For example, if you look on the NetHack subreddit, one of the current top posts is "Help me make it out of this group of enemies alive!", and everybody who comments has a different idea of what the best solution is.