A few basic ideas need to be understood to look at the question. I'm not an expert, but consider looking into some of these:
- gravity is the main thing holding our atmosphere in place. The 11km/s quote is the speed required for something to leave earth's orbit.
-gases can only push. They flow from areas of high pressure to areas of low pressure
- gravity exerts a force that stratifies our atmosphere into different layers, but generally speaking, more atmosphere is held closer to the earth than high up above it. Atmospheric pressure is greatest at sea level and gets exponentially lower as you gain altitude. This means that the higher up you go, the closer the atmospheric pressure gets to zero. At the boundary between earth's upper atmosphere and "space", atmospheric pressure is almost zero. This means that there is no pressure gradient to cause the gas to be lost to space
- earth is constantly losing parts if its upper atmosphere to space though, from interactions with the solar wind. If it wasnt for our magnetic field, we would lose atmosphere at a much greater rate (mars no longer has a magnetic field, which is believed to be a major cause of why it has such a thin -low pressure- atmosphere). Despite losing little bits of atmosphere, were constantly gaining 'new atmosphere's through volcanic eruptions, weathering processes, and biosphere interactions.
Again, dont take my word... I've undoubtedly oversimplified things. But dive in to some of those concepts and come up with an answer for yourself!