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Why could deuterium and lithium provide fusion fuel for billions of years?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

Deuterium and lithium are considered effectively inexhaustible fuel sources for nuclear fusion because they are abundant and can be utilized in a self-sustaining cycle within a fusion reactor.

The primary fusion reaction involves deuterium and tritium. Because tritium does not occur naturally in significant quantities, reactors use a "blanket" of lithium surrounding the plasma. When neutrons produced by the fusion reaction are captured by this lithium blanket, they generate more tritium, which is then fed back into the plasma as fuel.1

This process allows the reactor to essentially use its own waste products to create more fuel. Regarding availability:
* Deuterium: Can be readily extracted from seawater, where approximately one in every 3,000 water molecules contains a deuterium atom.1
* Lithium: Is an abundant and inexpensive element.1

Together, these materials exist in sufficient quantities in the oceans and on Earth to meet global energy needs for billions of years.1