Here's why:

* Helium is a very stable element. It has a tightly bound nucleus with two protons and two neutrons.

* Fusion requires overcoming electrostatic repulsion. The positively charged nuclei of helium atoms repel each other. This requires immense heat and pressure to force them close enough to fuse.

* Fusion products are heavier. When helium nuclei fuse, they can form elements like:

* Beryllium (Be): Two helium nuclei combine.

* Carbon (C): Three helium nuclei combine.

* Oxygen (O): Four helium nuclei combine.

These fusion processes happen in stars, particularly massive ones, where the conditions of extreme heat and pressure exist.