because that is their rest, or medium, position.
You have a muscles on the front and back of your wrist that extend or curl up your fingers.
The inside of your wrist completely curls up your fingers, the back/outside of your wrist completely extends your fingers. That half curl at rest is the middle ground between those two muscles.
The muscles that control your fingers need to be at rest at the point directly between fully open and fully closed, in order to allow ample movement both ways. If they were fully outstretched at rest, the closing of your hand would take twice the effort by those muscles and your arm would look considerably different.
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