Because the problem you are trying to fix is never a single frequency. Just like an EQ, the peaks and dips in the frequency response have a Q factor...it's either a wide (low Q) peak/dip or a narrow (high Q) peak/dip. Adjustable Q allows you to really narrow in on the specific problem you are trying to fix. Long story short parametrics typically have fewer bands than graphic, but parametric is more adjustable. So parametric is better at tackling specific problem areas in the frequency response whereas graphic is better at carving out the overall frequency response and tonality of the system. Both are effective, just at different things.