Because like all other debuff in this game (and all games) its suposed to do something itself.
In this game actually there is other "special" debuff applied by just 4 champions (2 of them rares, and 1 TOP because of that). Of course im talking about Frozen Banshee and Poison Sensibility. Its an "exclusive" debuff, but how is supposed, it make target weaker for all poisoners and not just for the one that put this debuff.
As simple as that.
But if u wanna take this to another level... lets go deeper!
Thea put Hex on basic, randomly in 2 hits (can go to the same target) - 5 turns debuff! (i think there is no other champion in the whole game that put so extense buff/debuff). This cant be resisted... but can be removed!
A2 - Atack all enemies and removes all Hex, increasing 50% attack by each Hex removed. (5 turns CD - Booked).
So, at this point, if u where lucky, u can double your damage for a nuke in your second turn (this is much more hard to set up in PVP). No sense to wait till a 3rd or 4th turn for maximize damage, because target probably clean debuff, die or u die by then.... And meanwhile... debuff itself do... nothing.
Ruel in the other hand... place Hex in all enemies with his A3 (this one can be resisted and removed) by 2 turns, and then in his next turn, with his A2, attacks with aoe, and give a 2nd shot to enemies under Hex effect (yes.. "effect"... what does it do this debuff itself again?... right... nothing!) and ignore 25% deff if at least 4 enemies are afected. Same point that Thea, u need wait to your 2nd turn... with all that implicate.
Even if u have both champions and want to make some sinergy... u need make Ruel fast enought to go twice before Thea remove the Hex, otherwhise Ruel´s A2 lose sense. C´mon, this is going worse each second...
Oh, almos forgot a detail.. Thea can be obtained by fragments completing DT in HARD mode!
So because this is an extreme "exclusive" debuff, just aviable for 2 leg champions (1 of them really hard to reach), and even with both, making a good sinergy is complicated, the idea that Hex itself has not effect at all is absurd.
Ironic thing, Ruel has a passive, wich could be simply Hex effect:
" Decreases the SPD of enemies under Hex debuffs by 5% and decreases the RES of those enemies by 10%. Also increases the damage enemies under Hex debuffs receive by 10% and decreases the value of heals they receive by 30%. "
And if u are using just Thea, this effec could be active. Or if in the future there are more champions wich ones can apply Hex debuff, all team can take a benefit of this.
Anyway, even that way the effect will be a crap, for been so "exclusive". The combo "reddef/incatk" or "weak/reddef" or other usefull combnations, which make some champions (much more than 2) top tier are easier to get, combine and setup, and get much more high results because this buff/debuff effects combinations, afect the whole team.