Many people don't know that supermarket bikes are often (not always) built from low quality materials, feature poor performing components, and are often assembled incompletely or incorrectly.
Building a bicycle requires great care. Imagine rushing through a final exam paper without proof reading or make sure errors are nonexistent and submitting that to a professor, that professor would grade you poorly.
A similar thing happens when building a bicycle, however, a professor won't scold you. What you will get is a dissatisfied customer and in a worst case scenario a potentially harmful or lethal problem.
Don't buy a supermarket bike, buy a bike from the a nice local bike shop (big or small). Odds are they will care much more about the build quality of the bike and you will be happier and safer.