From Dark and Darker Wiki
Weird question. "Because it's designed that way"
But if you are voicing your disagreement, I do somewhat agree with you. After all, they did make a stat for that, but for specific calss - Bard has Persuasiveness, that governs length of buffs and debuffs applied by bards to targets (that is additionally modifiet by target's own buff\debuff duration bonus). Wouldn't mind similar effect to have on other casters and even martials (Barb shouts, for example)