Food for thought - Rammus is a tank. Does Rammus need an ult that does a ton of AoE damage? It does 2/3 the damage of FIddle's ult per second but lasts 1/3 longer.
And you can ask yourself, why exactly does Rammus need this?
Well, he needs it because he is a tank. If a tank poses zero threat, there is nothing to prevent the Lucian or Jinx from just popping QSS and ignoring you. You need the high base damage in order to do your job. Sometimes it looks ridiculous on paper - like Rammus having 1560 damage in an AoE with a 240% AP scaling.
When they created an ADC that has 425 range and who does more than 200% damage from autoattacks at melee range (but starts the game at 70% AD damage at 425 range), yeah - you need something to help you survive.
Edit - And lets be real here - at max rank its 30 per stack, 120 max. In order to get the full 120, you need to Quickdraw 4 times. The bonuses last 4 seconds. His E has a 14 second cooldown, reduced by .5seconds per pellet hit.
With 40% CDR, his E is on a 8.4 second cooldown. At the very minimum, you will get 2 autoattacks in before his E is on cooldown again. So imagine a combat situation - You E in, Auto 2x, E again, Auto, Reload, Auto, E again, Auto 2x, E. At the very least, he needs to get 6 autoattacks and a reload off before he gets his full 120 resists. In a game where he isnt point-blank Meleeing you for the 3 second reduction and doesnt have 40% CDR, its considerably higher.
My Point - stacking mechanisms that arent completely broken or able to achieved in a quick amount of time just arent that useful. Look at Guinsoos - pre rework, it was useless even on champions like Kayle and Jax. Post rework it was broken, but only on champions that stack it quickly (Ahem - Kogmaw's 2x AS and 5.0 AS cap). A stacking 120 resists isnt as useful as Rammus' instant 120 resists. And that doesnt even take into account the thornmail-esque passive on Rammus' W.