Shielding and armor. Energy based shielding blocks out the force of the bullet, the faster and larger the bullet the more energy it drains, the smaller takes more but in rapid succession can deplete a shield just as easily. You know that white bar above your enemies health? Ya that's their energy shield, after that it's just armor plating protecting you from the outside forces. Also why it takes longer time for Titans to regen shielding cause theirs is stronger therefor needs more charge to recuperate. Also why warlocks armor regenerated faster, they have their magical force radiating from their body, therefore charging their shield faster.

My point is, the mass x velocity of a bullet from a gun is much higher than the momentum of a throwing knife. If a bullet cannot kill in one shot, would a throwing knife even do any damage to said shield. If we're using real physics, and a bullet doesn't kill, then a throwing knife shouldn't even do noticable damage.

actually a knife does more damage because the point of impact is significantly less, so any and all force put behind it is located in that tiny area