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Because the spear has already hit, no matter what sort of evasive maneuvers the target takes it will always reach them in the end.

Putting it in gaming terms, it’s an outrageous Noble Phantasm that scales its damage based on the enemy’s hit points.
For instance, if we assume that Saber has 100 HP, then Gae Bolg’s damage would be (Spear Damage) + (100)! As you can clearly see, being hit means certain death.