SnipSimilarly, a "hit" isn't whether or not you hit your opponent- it's about whether or not you penetrate their defenses.
The big weapons with Graze, then, are more efficient at penetrating armor (if I had a complaint about Graze, is that the mastery isn't applied to weapons that in real life were actually good at penetrating armor) due to their mass and striking power. This "armor-piercing" is reflected in the fact that, no matter how good your armor is, you can't complete negate these attacks.
Snip
Piercing defenders would make sense for things that have some sort of armour (plate, a tough hide, scales, etc). A character who blocks with a shield or dodges out of the way but is cut is hit. I wouldn't have issue with the unarmored person suffering from GFB or poisoned weapons, except that the attack did miss.As for your Green Flame Blade example, the spell probably should deal some damage on a miss (compare it with Acid Arrow, which does). The reason why comes down to two things- balance (if every flaming sword you find in game can deal some damage past armor, armor becomes less useful) and largely that "damage on a miss" was a concept introduced in 4e, and a few (but very loud) people trotted it out as another reason to hate 4e, because they felt it made the game less realistic (somehow). Thus when 5e was made, the concept was removed from the game (except for spells, where apparently it's ok, lol) to placate this crowd. Now, apparently, in 2024, they apparently found that the amount of players who hate it are an acceptable loss if they bounce off the game (I presume).
Melf's acid arrow sprays an area with acid, which is why did damage on a miss (not a lot of lab wear on the battlefield, alchemists not withstanding). Maybe flaming things should, but the rules don't seem to say they do, and you can quickly pass by a fire without being burned.