'cause her skill cap is high-ish, she's naturally tanky and can do some damage with minimal items. There for 1) it's easy to do well against lower skill people at low-mid levels which is where she's dangerous. 2) Later game she can soak a lot of damage.
Now for the skill cap part... assume someone follows through 1 and 2 but doesn't realize they need the rest of their team to win the game. Vi loses all forward momentum and crashes, hard, right at the other team's feet.
Or she carries her team out. Depends on the player.