Same reason you don’t poop where you eat – what went in (the reactor) is a lot different in look, feel and smell than what came out (the ground).
EDIT: New clear fuel,when originally mined, is at a steady state and live harmoniously with many other metals and minerals. Olympic dam is Australia for example, has uranium with 3 other metals (Gold, Copper, Iron ore). The fuel uranium has been turned from uranium ore into yellow cake, followed by lots of other refinements, so its fundamentally very different.
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