Creature theft effects and bounce effects make it so that running a card as suboptimal as Fowl Play shouldnt be necessary even if it has an occasional nice interaction, like turning an Elesh Norn or Griselbrand into a Chicken where stealing/bouncing it may have caused you to lose
Sure, the card is powerful and will be great in Commander, but its hardly an auto-include within every deck
If Mantle of the Ancients becomes attached to a different creature in response to its enters-the-battlefield ability, the creature it's attached to as that ability is resolving is the "enchanted creature," even though you used the original enchanted creature to choose targets
Prismatic Evolutions Price Watch - Sealed Boosters Want to pull these cards yourself
Sometimes you need a removal spell
Then creatures you control get +X/+X until end of turn, where X is the number of cards in your hand