Why do drive-reduction approaches fail to offer a complete account of human motivation?

I believe the answer is: People are sometimes motivated to increase rather than decrease their level of stimulation.

drive-reduction approaches is aimed to reduce humans' tendency to do a certain action/behavior by eliminating the need to seek the rewards that they might get by doing the behavior.

For example, when trying to make a person display appropriate behavior to lose weight, drive reduction approach would include things that would eliminate that person's appetite.