What makes the air too slow to get out of the way? I would have naively thought friction, and that if everything was frictionless the air would seamlessly move to replace the vacuum behind the aircraft, and thus no compression would occur.
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.
It's a common example in fluid mechanics textbooks to derive the flow of a zero-viscosity gas around a cylinder, you can get an exact solution e.g. here: http://brennen.caltech.edu/fluidbook/basicfluiddynamics/pote... . Note that in this example there is no drag, but there are two zones of high pressure in front of and behind the cylinder. Those zones would have higher-than-ambient temperature.