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Because the middle, ring, and pinky all share the same muscle group.

Try this: hold your hand out in a fist and extend your middle, ring, and pinky straight out. Now try to flex each of the three extended fingers individually without moving the other two.

Now try the inverse: make a fist and try raising each finger individually without moving the other two. It's physiologically impossible because the muscle used for extension is shared between those fingers.

(Edit: extension is impossible, not contraction)

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