To put it bluntly, air molecules aren't psychic. The only way they know what happens to neighboring air molecules is by running into them. At every-day veloci

To put it bluntly, air molecules aren't psychic. The only way they know what happens to neighboring air molecules is by running into them. At every-day velocities, this happens fast enough that we can treat it as though it is instantaneous. That the "vacuum" is a thing that sucks air in. At high velocities, this intuition breaks down. The communication time between molecules is limited to the speed of sound, the speed at which air molecules travel and bump into each other. If you are traveling faster than the speed of sound, (or several times faster, as is the case for re-entry), the air molecules don't have any information that an object is coming to them, because there hasn't been enough time for molecules hitting the object to then hit them.