Given the attack numbers of standard monsters, and the expected frequency of physical encounters (as opposed to magical ones), AC is a valid form of defense.
Are we talking about the same game here?
Core, CR 12 (because that's what I clicked on in the drag down box).
Frost Worm, looks like melee with it's one crappy bite. Oooh, that's because it has an at-will mass stun.
Kolyarut (inevitable), yeah it attacks, using touch spells.
Kraken, certainly melee. Still has SLAs through.
Leonal, thunder, thunder, thundercats roar!!! Just kidding, it's fireball, fireball, forcewall, dominate monster with a side of pounce/rake.
Purple Worm, melee! And no SLAs.
Roper, 50ft ranged touch attack that deals Str damage in preparation of pinning you and eating your corpse.
Only two of those care what your normal AC is. The rest target touch AC or can ignore it. That's 2/6, or 33%, or NOT half. Not at all.
And yes, things keep scaling more towards caster themed monsters. Maybe they appear like they should melee, or stab with claws, or bite faces off, but they are front loaded with SLAs and Save targeting special attacks that if you were to ever blow several thousands (and I do mean several here) of gold on your AC they can simply choose to ignore it.
It's just the problem that the standard numbers are woefully sub-par when pitted against the offensive power of even slightly optimized characters
Huh? This has nothing to do with PCs. That reads off like you're talking about a monster's AC. I was and have been talking about the player's AC.
But yes, an optimized character's offense greatly exceeds AC found in monsters.
And on the monster end, there is of course a huge problem. If you
don't give creatures a bunch of special abilities and quirks increasing their CR then you have to give them HD progression to achieve the desired CR. Problem with that is monster types gain a minimum of 2 HD per CR increase (many gain 4). Seriously doesn't take a rocket scientist to see the problem there. The system is totally designed to ignore AC, either by pumping more HD into a monster so it has insane HP, high saves/BAB, or giving it stuff to ignore AC outright but the rest of it's state block is manageable.