Why isn't there a fantasy trope that high level fighters learn to craft magic weapons and armor on their own?
I think the key is that the "farm boy who finds something special inside of himself and becomes a hero" archetype is pretty much the most common fantasy trope for fighters. Sometimes, it's just "common city guard who knows how to fight becomes a hero".
But rarely is the hero a blacksmith to begin with and most people think of blacksmiths as something you need to grow up your entire life doing and learning. You can't just decide to be one at 22 and have time to learn in between adventures. (Of course, as a side note, it's been done so many times that now people have no problem accepting that someone becomes a master swordsman after a week of training when they are 22, however)
That trope makes sense as an excuse for why high level fighters have lots of magic items. But most fantasy tropes originate in movies and books rather than from the games themselves. A lot of people want the game to follow the existing tropes in other fantasy entertainment. Aragorn, Boromir, and Legolas didn't craft their own weapons.
Failing that, the fighter can have a patron arcane weaponsmith -- like the way Bond has Q or Thor has the dwarves that made Mjölnir.
But this assumes a LOT of stuff in the game world. What if the PCs are playing escaped slaves of the Drow attempting to hide in the tunnels of the Underdark and do guerrilla warfare against their former masters. How are they suddenly going to get a patron without also ruining the tone of the campaign?
Failing that, the fighter is sorta like Indiana Jones or Lara Croft, specifically looking for treasure troves containing (or villians wielding) legendary weapons and armor, and these "side quests" are built into the adventure cooperatively with the DM.
The problem is that if your party comes with you, they are going to want to get a share of the treasure. You know how hard it is to convince the rest of your party that the magic suit of armor, the weapon, and the cool flying boot are all for the Fighter as a balance method for his class and the rest of you will just have to suck it up?
They are likely going to wonder why the DM didn't put equipment in there for the rest of them. Even if they know WHY...it still won't stop some people from role playing the fact that they think it's more fair if they split all the items up, that way everyone else can sell the items and buy what they want.
That's the problem with using equipment as a balancing factor. In character, there is no good reason why a bunch of people risking death together would be handing over the majority of their treasure to one person over the others.
Well, unless you can't sell magic items. In which case, that sword is ONLY useful to the Fighter, might as well let him use it.
Mythic fighters wouldn't need to rely on magic trinkets, but why don't mundane fighters reliant on magic items/armor take control of their own fate, instead of passively hoping for magic gear?
I think the entire point of going into dungeons and fighting monsters IS to take control of their own fate and find powerful magic items to use.
And the average dungeon gives them 6 suits of armor and 12 weapons anyways. So, there's no hoping involved.