Interesting. The last sentence on the Wikipedia page for helium:
"Helium is a finite resource and is one of the few elements with escape velocity, meaning that once released into the atmosphere, it escapes into space."
So how does that work?
At a given temperature the typical velocity of a gas molecule depends on its mass. The lighter, the faster.
Helium is the only gas molecule that is stable in the atmosphere and has a typical velocity near the top of the atmosphere that is faster than Earth's escape velocity, so it slowly diffuses up to the top and then is gradually lost to space.