The fiery re-entry of spacecraft has been a staple of spaceflight since the beginning, making ablative heat shielding a necessary component of any craft wishing to return to Earth intact. This is the result of the shockwave caused by the high-speed encounter with the upper atmosphere, in turn dictated by the angle of re-entry (~40 deg for the Space Shuttle). The literature on this topic always says that too shallow of an angle would result in the craft "skipping" off of the atmosphere. This seems to imply that the atmosphere has a "surface" similar to the ocean, which is certainly not intuitive for a gaseous fluid.
So my question is, why is it impossible to gradually, gently deorbit using a shallow glidepath? Is there no combination of aerodynamic surface & thrust profile that could make this happen?