The atmosphere is 100km, but it's not equally thick everywhere. At the top of Everest, the air density (and pressure) is only one third of the sea-level. Because pressure is directly related to the weight of the air column above you, this means that a rocket launching from Everest only needs to go through one third of the air that a rocket launching from sea level would. Less air to go through, less energy lost to air resistance, less fuel consumed.
Further, because the air is less dense, the dynamic pressure acting on the rocket would be much lower, which means the rocket doesn't have to be as strong to withstand it, so it can be even lighter, resulting in more fuel savings.
However, even with taking all this into account, it's nowhere near worth it.
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u/Triqueon 11d ago
Yes, which is why the American Spaceports are in southern states, and the ESA launches from French Guiana.