A quick photo primer on why the cheap knives so tough.

Large carbides, clumps thereof, and grain boundaries, weaken steel, flaws in a diamond or knots in a structural board or pole.

The following are photo-micrographs at identical magnification of the cleanest powdered steel I could find (listed by alloy content, not name brand, and I am not messing with looking up), then 440C, then D2, then Sandvik 12C27.