Shortest answer . . . it doesn’t currently exist in high enough concentrations to achieve the critical mass needed for an energetic reaction (heat) much less explosion. Critical mass is when the uranium atoms are close enough together that if one goes, it starts a chain reaction and they all do. Before it is mined, refined, and piled together, those single random decays just wander off into non-uranium matter and lose their energy before they can start something.
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