An injury that leads to tons of blood loss is very traumatic for the body. A needle that extracts some blood barely registers. These aren't equivalent things.

People needing blood often need multiple transfusions as they are actively loosing blood. The transfused blood buys surgeons time to find and stop the bleed(s). A person donating blood is losing a controlled amount of blood that is safe to lose.

If you lose too much blood organs and tissue start to die due to lack of oxygen. We can't reverse this damage and depending on what died our body is also limited or incapable of repairing the damage too.

Transfusions try to maintain enough blood in the body so the heart and lungs or a bypass machine can keep your body saturated with oxygen. A person donating has no risk of this.

Your body can't just magic out liters of blood. Where would it come from?