Plenty of "removal" cards that can deal with your opponent's creatures
The fact that their two innate keyword abilities, first strike and deathtouch, work so well together is a lovely bit of mechanical serendipity that makes the duo nigh-unstoppable in combat
MTG: Final Fantasy - Chocobo Bundle The Chocobo Bundle comes with 1 Chocobo Booster, 10 Play Boosters, 1 alt-art promo card, 1 Scene card, 32 basic lands, 1 click wheel, and more
It gives the proxies a more authentic feel
As well, once Ovinomancer was active, it was nigh-unkillable itself, since returning it to your hand is part of the cost and not effect
One advantage that items have over characters is that they can be used as soon as they are summoned, as they do not need the ink to dry on them