On top of what Wolf said, swords aren't made to deal with the stresses some knives do.
Though some swords DO have full tangs, such as Messers and (arguably) Katana, most swords have a hidden tang. This is acceptable because a sword is not made, nort expected, to deal with the shock of striking hard targets.
A cutting sword, when used as it was designed, will slide through flesh and (live) bone cleanly and pass through without a sudden stop. That's pretty much it's ONLY function: cutting live, mammalian tissue. This is in stark contrast to a knife, which is needed as a general use tool to slice, carve, chop, baton, etc.
Though a sword can still be considered a tool, it's use is VERY narrow. A knife should, in comparison, by more sturdily engineered to deal with the varying uses.
Note: When I refer to the function of a sword, I mean a REAL sword. Lately there's been a slight trend for over-engineered swords, like the SwampRat Wakizashi and Cold Steel Katana line, which are made to deal with stresses no historical sword design would or should. Such swords aren't made for sword-arts, but for brutal chopping, non-practitioner use, and outright abuse by the ignorant.