Void Staff is better when you compare it to Cleaver or Last Whisper as single items, but not when MPen items and Armor Pen items are all put together. Armor Pen Stacking is just significantly stronger than Mpen stacking. For good reason, but Armor should still matter against an AD opponent and it simply doesn't if they have so much Penetration. If you ask me, they should rename all Armor Pen options "Unique Passive - Lethality" so they have to pick between %Reduction (good vs squishies and tanks but not as good against either than %bonus pen or flat Lethality), %Bonus Pen (good vs tanks, useless vs squshies), and Flat Armor Pen, and then find some other way to control an Assassin's snowball.
Morello's Grievous Wound only activate on target's 35% HP or less, while LDR's activate on dealing any physical damage to the target. Either Mages can't get the target's HP to 35% or less in the first place (vs tanky targets like Swain, Vlad, Warwick etc), or it doesn't matter because a Mage will have already killed them (Fiddle, Lifestealers, etc)
Magic Pen stacks the same way, but it doesn't matter because Armor Penetration items add a layer that Magic Penetration items don't. And that is the reduction layer, since old Abyssal Scepter was the only item to reduce MR at all. So a champion gets their Armor reduced by 24% from Black Cleaver, then their assailant ignores 50% of their bonus Armor, and then ignores up to 54 of their remaining Armor. I'm not willing to do the math since I'm unsure of how the reduction interacts with bonus/base armor values, but a target with 150 Armor will probably be sub-30 Armor after all that. Meanwhile, a target with 150 MR would be at roughly 67 MR after Void Staff + Sorc's/Liandry's. I did that math because its a lot easier.