Here's why:

* Most common fusion reaction: The most common fusion reaction in stars, and the one that powers our sun, is the fusion of two hydrogen isotopes (deuterium and tritium) into helium. This process releases a tremendous amount of energy.

* Other possibilities: While helium is the most common product, other fusion reactions can produce different elements, including:

* Carbon: Fusion of helium nuclei can produce carbon.

* Oxygen: Further fusion of carbon with helium can produce oxygen.

* Heavier elements: Fusion reactions can also produce heavier elements, though these become increasingly rare as the elements get heavier.

In summary: While there are other possible products, helium is the most likely and abundant product of fusion reactions, particularly those occurring in stars.