Laxatives are only going to get rid of what is in your digestive tract, it is not going to pull any fat off of your body.
Most people when they want to lose weight, don't just want to see the number on the scale go down, they want to get rid of the fat that is on their body. In order to do this, you need to create a caloric deficit and sustain it over time.
The longer you sustain it, the more weight you lose.
So basically, the primary reason laxatives are not effective for losing weight is because they only help you empty out whatever weight you're carrying within your digestive system.
To really lose weight you need a solid diet/exercise plan.
