This has been bothering me ever since the first time someone mentioned it in a thread about plot holes.
Why?
Is it because the people like challenges a la Goku? If so, doesn't that mean everyone in the universe is just a supreme d1ckhead?
And if nobody has thought of it, doesn't it mean everyone's just stupid?
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Not within Shenron's power. He can't grant wishes to defeat any being that is stronger than his creator, making anyone beyond Dende out of the question. And the protection would have been gone anyhow once Piccolo died and Dende wasn't around, leaving a window of vulnerability any time Earth had no Namekian guardian.