Why does Forsaken Wastes - a faction comprised largely of skeletons, zombies, spirits, liches and other undead horrors - have so few Fearless units?
the same reason skeletons can bleed and you can equip boots of de'lim on colossal boa. because you have to compromise theme so the game isn't massively complex, imbalanced, and inconsistent. and because it's a game and not real life
why can constructs be poisoned why can snakes equip boots why can relics be eviscerated why can voils be blinded (bats hunt using echo-location) why can a unit with colossal be shrunken and still have colossal why can you charm a plant/construct why is elven prodigy affected by Impair Vision? Because it's a game, not a simulation. you don't need everything to make 100% sense
The fact that there are other examples of stupidity doesn't make them any less stupid. ... Yes, how stupid it would be if a game actually made sense.
Pox has other priorities than sticking to strict semantic logic. It aims to provide a competitive and versatile environment, not one obsessed with consistency of detail.
The undead are afraid of what is holy or that which has abundant life. So fearless would not make sense as they typically run away from light and power.