There's certain feel/reactions that these rules want to invoke, but that I think they inevitably don't because it just doesn't jive with typical play behavior.
I.e. if a monster is resistant to slashing and piercing, you want the PCs to scream "Crap", scramble for improvised weapons, scratching out victory, or they all rally around the one character with the appropriate damage type, aiding him and the spotlight goes to him. This is evocative and found in fiction. But most players will just grit their teeth and grind through the encounter using their weaker weapons while glaring at the DM.
If a monster is resistant to all but magical items, you want players to feel fear, to rally around the one guy with the magical item, try something creative like wrap a club in a magical cloak, or you want them to try and power-attack, praying they breach the creature's hide. This is evocative and found in fiction. But most players will grit their teeth and grind through the encounter with their weaker weapons, while cursing the DM for throwing a monster that's hard to kill because the Dm hasn't given them adequate magical weapons.
If a monster cannot be destroyed via conventional means, you want players to look for unconventional methods like say, collapsing a tunnel on top of a monster. This is evocative and found in fiction. Not only "collapse the tunnel" is very unconventional way of thinking when approaching an adventure (Well we need to get through that tunnel when it's over, it's never been done before in any of the players careers, etc), but there are no clear and concise rules to handle "collapse the tunnel", so the Dm has to just hope the players get the idea. Instead, most players will grit their teeth and keep trying, and some characters will likely die before they realize the monster is "unbeatable", and then they get mad at the DM for throwing something unbeatable at them.
(Yes, your group may commonly try to collapse tunnels or rally around the one character with the appropriate damage type, but when designing rules, the designers must consider how most groups are going to play the game. Doing something to reward what most do not do would be enforcing a certain playstyle.)
Then there's the issue of rare item types, or at least item types that only effect a small subset of monsters. Silver is a good example. In 4e, the only monster that needs a silver weapon are the uncommonly encountered lycanthropes. So, the only reason you should ever get a silvered weapon is if you need to fight lycanthropes (3e was better with this in that a few others had silver weaknesses, but it was a small list). The same goes for Good aligned weapons, etc (and don't get me started on Law/Chaos). Adamantite was useful only for constructs (which don't appear that often anyhow).
You also come into the problem of "Ok x should kill monster y, but how do we use it?" like a wooden stake. You should be able to stake a vamp. But is that a called shot? A critical (rolled while wielding a 1d4 improvised wooden dagger)? A coup de gra- oh right, vampires turn to mist when reduced to 0 HP.
So you want to evoke a certain feel derived from fiction, but what you get is the opposite to ease of play. You wind up with melee characters carting in a golf bag of weaponry, checking their inventory to make sure they have an item of that damage type/property. Switching weaponry leads to a few minutes of changing their to-hit and damage calculations because they probably have feats tied into certain weapons, and those weapons have different enchantment effects/damage die. It doesn't feel exciting, just feels like refitting lower-quality tools.
Or you just play a caster and only worry about energy resistances and SR.
I consider most to encompass 60-70% - and I would not characterize 30-40% as a few singular exceptions. Had I meant "almost everyone", I would have said something like "nearly everyone".
