To do with density (He in lower atmosphere would get displaced by heavy gasses 'falling' down to the surface?) and KT physics (in a group of particles the distribution of energy is uneven - some have more, others have less. The particles with more energy would be more motivated and able to escape. Similar to how water can evaporate without boiling. My best guesses. Not studied physics for a decade or two.
The earth does lose a lot of helium, doesn't it?
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