Not only it is blocking blood flow as others have pointed out, but it can be blocking flow of deadly infection, such as if a bowel has been perforated.
Also, not all knife blades are straight. Pull up on the wrong blade, and you will do additional damage to the person on the way out. Medical personnel can do imaging to determine what kind of blade it is, which organs and vessels it threatens, and carefully extract it with clamps in hand and with antibiotics at the ready.
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