Why is there so little Helium on (inside) Earth? According to this article, there is a large quantity of Helium in space (stars), but very little is accessible to us on Earth:

Helium is abundant in space, where it's produced as a product of the
fusion reaction inside stars such as the sun. The naturally occurring
helium on Earth, though, comes from a different sort of process.

This might sound silly, but I could not find information about why we have so few Helium atoms "trapped" inside the Earth from when it was formed, since it was abundant in space during the birth of the planet.
Note: I am mostly referring to Helium-4, as the shortage seems to be different depending on the isotope as explained here.
 A: Helium is not very dense. So whenever it enters the atmosphere it floats up to the top. In the upper atmosphere, the temperatures are such that a decent amount of the helium has a thermal velocity that is larger than escape velocity. Such helium will escape to space and leave the Earth. This is why there is not much helium here.
The same argument as above would apply to hydrogen. However, hydrogen is very reactive, so it easily binds with other heavier elements, like oxygen, to form compounds that are not so prone to escaping. Helium, being a noble gas, does not do that. So the above argument works for helium but not for hydrogen.
A: The earth does contain primordial helium that was trapped when the earth was formed. It is impossible to distinguish between helium-4 produced from alpha decays inside the earth and primordial helium-4, so studies of primordial helium look for helium-3, most of which is primordial.
According to a recent study, about 2 kg of helium-3 leak out of the earth every year, and it is estimated that the earth contains $10^{10}$ to $10^{12}$ kg of of helium-3.  (The wide range is because it is hard to estimate both early degassing of the the hot earth and current diffusion rates from core to mantle to the surface.) Given that helium-3 is about 300 parts-per-million in cosmological helium, this suggests that the earth's total primordial helium content could be as much as 1016 kg, or about 10-9 of the earth's mass of $6\times10^{24}$.
As the answer from @Dale notes, all helium - both primordial and radiogenic - that reaches the earth's atmosphere tends to escape relatively quickly.  Some helium atoms at the top of the atmosphere have speeds above the earth's escape velocity and are lost by the simple thermal Jean's escape process, but polar wind escape is more important.
