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Why do astatine and tennessine not occur naturally?

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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. — halogen Modern version of the periodic table of the elements.

Astatine and tennessine do not occur naturally in significant quantities because they are highly unstable radioactive elements with very short half-lives.1234

Astatine: While trace amounts of astatine exist in Earth's crust as a decay product of heavier elements, it is one of the rarest naturally occurring elements, with less than one gram estimated to be present at any given time. Its isotopes are highly unstable, and their rapid radioactive decay prevents the element from accumulating in nature.14
Tennessine: As a transuranium element with an atomic number of 117, tennessine is entirely artificial. It does not occur in nature because its isotopes are not stable; any quantities produced decay too quickly to accumulate.32

Because of this instability, both elements must be synthesized in specialized facilities, such as cyclotrons, by bombarding target materials with heavy ions.234