Why can thorium and uranium still be found naturally on Earth?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

Thorium and uranium are found in nature today because they possess isotopes with extremely long half-lives, on the order of billions of years. These isotopes, specifically thorium-232, uranium-235, and uranium-238, are considered primordial, meaning they have persisted on Earth since the planet's accretion.1

While all actinoid isotopes are technically unstable and undergo radioactive decay, these specific isotopes decay so slowly that they have not yet vanished. Furthermore, other shorter-lived isotopes of these elements are also found in nature because they are being constantly replenished through the decay chains of these long-lived parent isotopes.2314