Sooo why not give him a CC ability that actually makes them and causes him to be more threatening.
Because taunt's are really lazy design.
Taunts are mostly for making PvE experiences a little more interesting by allowing some control over an otherwise rigid AI.
The ideal scenario with creating solid tanks and fighters in a PvP environment is to make them threatening enough or problematic enough that other players are torn between focusing them over other units. In essence, causing them to generate aggro in a real-world sense rather than using a fairly weak and flawed simulation of the same.
This also allows for greater depth and a greater skill ceiling for tank players, because you have to actually read your opponents and make the plays to generate aggro, or "taunt" enemies, in the real-world sense.
A solid Singed player will get people to chase him. Solid Singed players are actually pretty amusing to watch, walking the fine line between life and death that lets them bait opponents into their poison rather than succumbing to overextending/suicidal misplays.
TLDR: a taunt would take away from the depth in his kit rather than add to it.