Why doesn't Oblivion Orb give Grievous Wounds? AD Champs get the effect for much cheaper
800g
2600g
???
???
#3000g 
I'd gladly trade away 15 penetration on a component if it means I can deal with the absurd healing in the game.
800g
2600g
???
???
#3000g 
I'd gladly trade away 15 penetration on a component if it means I can deal with the absurd healing in the game.
Because only brainless AD auto-attackers and punching-bag tanks deserve it I guess.
God forbid we hand some to the most skill-shot reliant class in the game...
75 percent of champions can't effectively build that armor pen item.
Every AP champion can build Orb
It's obvious they don't want to give AP as easy access for a few reasons. For starters there tends to be a lot more AoE damage associated with AP damage. Two, there are a ton more AP DoTs than AD and they're easier to apply. Factoring in a single DoT could inflict GW for 6+ second. That's not even including a DoT with Liandrys added. You could easily achieve 10+ seconds of GW with a single DoT. Actually, any AP damage in the game becomes a minimum of 6 GW with Liandrys. A third reason is you could combine the cheap AP GW item with something like Cinderhulk to create permanent GW auras. This goes for any damage aura in the game really. I could be mistaken but I don't think there is a single AD damage aura in the game. Bramble is already bad enough but at least you can choose to not attack them if you absolutely need the healing.
So if you factor in all the possible abuse cases, it makes a lot of sense.
Most AD champs face a lane where they benefit from grievous wounds much earlier. Particularly, the top lane. Most mages don't usually need it until the teamfights start, and at that point, you would have long built the full item if it was what you wanted.
who the crap are you worried about healing in the mid lane? Midlane doesn't have to deal with excessive sustain the way that top and bot do.
well who the fuck build a last whisper first item , and also mages are busted as fuk right now
No one wants to buy Executioner's Calling, unlike Morellonomicon and Thornmail.

Because executioner's is a trash item and nobody rushes it.