Realtalk: because that would make certain abilities completely busted and unfair. How would you manage to balance the various creatures that have instant-kill chances? How the heck would you buff Mite Gravedigger, whose ability already makes it immortal so long as it isn’t your last creature?
Off the top of my head I can recall a dozen creatures that already dominate the lategame with their abilities - Raven Acolyte, Necropolis, Deity, Mortarsman, Bone Reaper, Pegasus, Frenzy Ghoul, Nightmare Mummy, Mouth of Hell, Topaz Paragon, Doom Devil, or Bile Slime. These are unbuffable - they already own the top-tier.
Likewise, a Nether Royal Phoenix having a 75% chance to revive or a Nether Overseer having an 80% chance to revive its teammates wouldn’t be better for the game, just far more frustrating.