This is actually a common misconception. Thief's Knife exists to severely annoy Thieves just by existing. Originally the method through which Thief's Knife was obtained achieved this goal, and now the profound wretchedness of a level 75 weapon achieves this goal.
There was a time when neither applied and Thief's Knife was only "mildly off-putting". For the micro-nihilist who designed Thief's Knife in order to watch some small corner of the world sort of smolder just a little bit, those were dark days indeed.