This is a combination of 2 reasons. First, gravity. The gravitational force keeps air molecules close to the earth. This reason would be enough in an ideal place, but not when there is the sun nearby. The sun creates the solar wind: trillions and trillions of charged particles traveling with great speeds arise from the sun and crush on the planets, every single second. Mercury has no atmosphere because of the solar wind since all the atmosphere it once had has been destroyed from the solar wind. Even Mars has less atmosphere than the earth because of the solar wind, which is a reason that even if life existed once in Mars, now the thinner layer of air cannot allow it.
Earth wouldn't have the atmosphere capable of holding life too, if it weren't for the second reason: magnetic field. Earth's magnetic field stops the solar wind from penetrating into the atmosphere, and therefore allows its existence.
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