Now let´s compare the two of them.
Malphite's passive gives him a shield based off of his max HP, that can easily be lost and not easily regained.
His Q is a targeted MS steal i.e. slow+MS boost.
His W is passive armor increase and tiamat passive when activated.
His E is an AS reduction that forces him into an armor stacking build in order for it to do anything.
His ultimate is the only reason why the purchases Abyssal Scepter and while it is a teamfight changer, it still keeps things 5v5.
Poppy's passive is also a shield that scales with her health but is easy to get because somehow Riot thought that giving it for free when she last hits with the buckler is fair.
Her Q is an area slow with %maxHP damage.
Her W increases both her resistances, doubling the increase when she's is below 40% HP. And it stops dashes.
Her E is a displacement and if positioned properly, a stun.
Her ult is also a displacement, one that changes teamfights around easier than Malphite's.
Now consider which keystones they take.
CoC for Poppy, Grasp for Malphite.
Now consider dash matchups, in which 3/4s of Poppy's abilities proc CoC, one without any effort.
Malphite takes Grasp because that´s all he can take, unless he goes for the cheesy TLD in some matchups, which is unsurprisingly sup optimal, at best.
Plus, Malphite builds mostly tanky with the exception of Abyssal. And he does so simply to give his ultimate, the only actually important ability in his kit, some extra damage. If he took CoC and somehow, by some miracle, hit 5 people with his ultimate, sure, he'd get a big shield from the proc, but it´d mean nothing because he can´t follow up on that.
Poppy's nerf is well deserved, as is the current extreme nerf to CoC on PBE's 7.2 patch cycle. Tanks and bruisers with a lot of reliable hard CC and shields, i.e Poppy and Nautilus, should not be able to get high returns from CoC proc. It´s the reason they became overbearing. And with an exception to that lot of hard CC rule, it´s why Darius sneaked back into meta when he shouldn´t be in meta.
The reason Malphite's ult has high base damage with 100% AP scalling is because other than on maybe URF, no one, and i repeat, NO ONE is going to build full AP on him.