A champion made to COUNTER ADC is halfway dead because of last whisper.
isn't really made to counter ADCs directly. His taunt is much better used for it's capacity to silence abilities on mages or assassins over encouraging an auto-attacker to auto-attack you.
In the general scheme of things: defense counters burst, burst counters sustained damage, and sustained damage counters defense.
Rammus is not on the right side of that to be an effective marksman counter. His strengths vs. AA damage mostly nullify them countering him rather than the other way around.
He is a fantastic assassin counter though.
Also: as a CC tank his roll also isn't being an anti-carry, it's initiating team fights and locking down priority targets, with the back-up plan of counter-initiating fights and peeling for his allies.
Rammus's main objective isn't to kill.
last whisper and void staff ruin league. Reason being ... say you have 300 armor on rammus. Rammus comes in for a gank with q right before he gets to her she net traps away and his q fails to get in range, rammus is now slowed and not in range for taunt. Caitlin does 3 auto attacks and rammus is already at half health.... WTF?!?!?!
195 armor (300-35%) is still ~66% damage reduction vs. physical.
To get to the 3-major item point where marksman typically pick up Last Whisper, and to amass a bonus ~220 armor, you're going to have at least 3000 HP as well.
Which means for Caitlyn to shave off 50% of your HP in 3 auto-attacks: her pre-mitigated damage would need to be 1500 per hit.
Or 600 total AD with 3 critical strikes from IE in a row.
Not exactly impossible with 6 major items and the right supportive buffs. But you should have considerably more than 300 armor and 3000 HP at that point as well, least you're expecting to retain any semblance of presence with only 3 major items.
You're overestimating the power of those 2 items. By a lot. Also misreading Rammus's roll.
Picking a tank isn't a promise to be invulnerable, it's just a play-style akin to using mobility or range to survive long enough to get your job done.