Actually, some fractions of the atmosphere do "boil off": https://en.wikipedia.org/wiki/Atmospheric_escape https://en.wikipedia.org/wiki/Atmospheric_escape Es

Actually, some fractions of the atmosphere do "boil off": https://en.wikipedia.org/wiki/Atmospheric_escape https://en.wikipedia.org/wiki/Atmospheric_escape Essentially, because the distribution of thermal energy has a fairly long tail, it is possible for some lighter particles to reach escape velocity. The hotter the atmosphere, the more (and heavier) particles escape.