because pins are a method of securing prey that does not necessarily mean instantaneous death. herd mentality about "pinslop" aside, its better than the alternative options that have been tried out before for these playables or similar ones
take rex for example, the crush pin takes up the role of allowing adult tyrannosaurs to hunt smaller faster prey items if caught in an ambush, which (other balance issues aside) lends rex to being far less one note in how it interacts with things. The previous option that existed for rex in this regard can be seen in legacy, in addition to the other dinosaur survival games, where if a rex bites you, your leg breaks instantly and you are dead.
Pins allow for punishing a player who has not managed their stamina well, while also making it so rexes interactions with things in its own weight class dont follow the exact same motions as its interactions with things smaller than it
allo follows a similar path but inversed, where its pounce fills the same role as it does for omni, allowing it to hunt large game without being at constant risk of death every time it wants to do damage, and its pin fills the same role it does with rex, just sub out leg break with bleed. (that being said its group pin is horrifically oppressive)