Amumu's strength isn't his damage. It's the fact he can actually miss a champion and still get off his kit.
Don't have a line of sight on a champion? Just Bandage to a nearby minion, and Wham.
You're overestimating his damage though. I can do more with Morgana, and I can clear faster with her. Or rather, once I'm at a camp, since Amumu can just go through walls.
They just play differently.
Amumu chases.
Morgana waits in a bush using predictive play knowing where someone's going to be next.
Also, Amumu doesn't have a spell shield.
Let me put it this way: I have yet to be countered by ANY jungler other than Lee Sin on Morgana. Amumu has counters. Amumu you can just build Mag Resist to, and certain melee skirmishers can kill him in an outright duel once they have Armor pen/Sustain.
Morgana on the other hand, gets to block their CC, and keep running, possibly double CC them back, and refresh her kit. If built correctly, she can totally outlast and out duel an Amumu.
I'm going to let you in on a little hint to one of the things I do sometimes when going Morgana jungle.
Sometimes I grab a
and/or a
.
Yeah Yeah. I know. Lowest Auto attack range mage in the game, and she has her own sustain.
BUT...... Nashors and Gunblade massively speed up her jungle clear, simply by dropping the primary mob low faster, elevating her W damage.
Morgana is also good at creating a captive audience, where they are 200-300 range from her for 3 seconds, and she can be Ulting them at the same time. If as a jungler you build like this, it not only gives you substantial sustain, and AP values, but it unlocks unexpected damage avenues on a champ everyone expects just to be a full AP, behind, low damage "troll pick." It's just something to put her over the edge. Kind of like how people playing her Mid are picking
as supplementary damage.
Granted, as a laner, I never buy Nashors. But I do occasionally buy the Gunblade, and it changes fights significantly. No one expects a Morgana to kill a Rengar/Jax/Kha in 1 second