While studying about Maxwell-Boltzmann distribution (through a chemistry textbook) I came across the fact that molecules can posses any speed (lower than $c$). This makes it obvious that there are many molecules that have speed greater than escape velocity of the earth. It is then reasonable that some amount of molecules are present at the outer portion of the atmosphere would posses such velocity and hence escape away. This process would continue in a similar fashion so that in a long run a large amount of our atmosphere would have she'd away. Given the fact that earth is 4 billion years old my question is:
Why hasn't such thing happened?
Why do we still have our significant portion of atmosphere left(if such process had happened)?
Please do correct me wherever I go wrong.