Here's why:

* Isotopes: Helium has two naturally occurring isotopes: Helium-3 (3He) and Helium-4 (4He).

* Abundance: Helium-4 is much more abundant (99.999863%) than Helium-3 (0.000137%).

* Weighted Average: The average atomic mass is calculated by taking the weighted average of the masses of each isotope, considering their relative abundance.

Therefore, the average atomic mass of helium is closer to the mass of Helium-4, since it is far more prevalent.