The impact velocity is very high and so the effect is more like a sudden detonation than a gradual excavation. Bear in mind that the size of the crater is much larger than the diameter of the impacting object. There is a lot of kinetic energy in a 100m diameter object going at 20 km/s and this is released very quickly in a small volume some way below the surface. The resulting shock waves are spherically symmetric until they break the surface and this means that, with the exception of very oblique impacts craters will be circular.
British Astronomical Association
Supporting amateur astronomers since 1890