Lets say that, for example, you draft the following Selesnya Midrange deck (list taken from the Turn One Magic blog, a nice Cube blog that talks about decklists and theory): Creatures (12) 1 Llanowar Elves 1 Birds of Paradise 1 Fyndhorn Elves 1 Phantom Centaur 1 Qasali Pridemage 1 Acidic Slime 1 Baneslayer Angel 1 Stoneforge Mystic 1 Mirran Crusader 1 Primordial Hydra 1 Fiend Hunter 1 Wolfir Silverheart Lands (16) Spells (12) Take four nonland cards and pretend that theyre replaced with Bonescythe Sliver, Gemhide Sliver, Manaweft Sliver, and Chameleon Colossus
Now Mew is at your command
In short, they just dont shine correctly
Attendant of Vraska Baloth Null Belbe, Corrupted Observer Dreg Mangler Erstwhile Trooper Glissa Sunslayer Glissa, Herald of Predation Glissa, the Traitor Golgari Rotwurm Grizzly Ghoul Host of the Hereafter Jarad, Golgari Lich Lord Llanowar Dead Lotleth Troll Marrow Chomper Molderhulk Moldering Karok Necrogen Rotpriest Polukranos, Unchained Putrid Leech Rhizome Lurcher Shambling Shell Skullbriar, the Walking Grave Slitherhead Sludge Titan Storrev, Devkarin Lich Underrealm Lich Vulturous Zombie Wight of the Reliquary Woodwraith Strangler