I'm not an expert, but it seems like harnesses are designed to be quite difficult to remove, as accidentally doing so (or the strong downwash turbulence of an open flight doing so) in an open-door flight would be almost certainly fatal. The knife is kept as an emergency alternative to allow for faster release than going through multiple buckles, not the primary method.

If I recall correctly, parachutes use a similar thing - in normal circumstances, you simply unbuckle after landing, but in a foliage or water landing, you can cut yourself out of the straps.