why is there still radioactive material on earth? Why hasn’t it all decayed?
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The half life of some of these materials is insanely old. The half life if uranium-238 is about 4.5 billion years. That means about half of the uranium here on Earth is only half of what the Earth started with.
Additionally, other things can decay into radioactive elements. Radon-222 only has a half life of 3.8 days, but more is constantly being made
U-238 -> Th-234 -> Pa -> 234 -> U-234 -> Th-230 -> Ra-226 -> Rn-222 -> and then a chain of polonium, lead, and bismuth that eventually becomes a stable lead-206
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