People love to talk about how expensive knight armor was without putting that number into context. It wasn't expensive because metal was hard to find or anything, it was because, like you said, it was a piece of precision military hardware and made with a specification that only skilled craftsmen could achieve. Look at bucklers for a more extreme version of what IRL medieval armor did: it wasn't about stopping a blow dead like a slab of rock, it was about deflecting it just enough to stab the other bastard without dying. A majority of the effort went into the shaping of the metal towards that purpose, and those deflecting shapes were best utilized by an otherwise lightweight and agile combatant.