Getting muscular requires a delicate balance between a lot of lean muscle mass and little body fat. Lifting weights, however, has little effect on any of those two factors.
Lean muscle mass
Muscle size increases due to the effects of resistance exercise, which produces microscopic tears in the muscle fibers. During the subsequent rest period, the muscles are repaired and reconstructed being larger than they were originally. Unless they are used for long periods, say 100 repetitions or more, light weights do not offer the level of resistance that is required to produce this effect.
Low body fat
You can lose body fat by burning more calories than you consume. For two reasons, lifting light weights does not have much effect on the number of calories you burn. First, it does not take a lot of energy to accomplish this task and therefore you burn fewer calories. Second, lifting light weights does not increase muscle size and therefore can not increase your basal metabolic rate, which is the amount of calories you burn while sleeping or sitting on the couch.