This is a question we get at the shop all the time, and the answer is pretty basic. Quality!!
It boils down to the fact that there are no bike manufactures in the world, they are all bike "assemblers." No bike company has the ability to make the entire bike, they would have to own a frame factory, a tire factory, a gear factory, a pedal factory, a seat factory, etc. etc. etc.. No one bike company can own all the different factories it would take to make all the parts of a bike. Instead they are bike assemblers, they buy different parts from the different factories to assemble their bikes. Now each one of these parts factories has a whole catalog worth of parts, from the very cheapest to the most expensive. A tire factory will make 100's of different tires from 10 cents to 100 dollars. In all sorts of sizes, colors, and quality's. And this also goes for seat companies, pedal companies, etc. etc.
Now the average bike has over 350 separate pieces and if you go to Super Mart and buy a $99 bike, basically nothing on that bike can cost more than 28 cents. How long do 28 cent parts last?
The above picture is a pretty good example. The "single wall " rim on the left is typical of big box style bikes. And the "double wall" rim on the right is bike shop quality. Both of these rims are made by the same rim company. Twice the metal and much stronger. Another example would be frame material, steel frames at big box and aluminum at bike shops. Making bike shop bikes lighter. And it goes on and on with every single part. Big box bikes do look just like bike shop bikes, but the quality of the parts is no where near equal. So the better the bike you buy, the better it will ride and the longer it will last.
Another comment we get is, Why should I buy a good bike when they're just going to out grow it? Our answer is simple, If you buy a good bike and trade it in and buy a good bike and trade it in, by the time they go off to college, you'll have spent less money than buying and throwing away, buying and throwing away.
Please stop by and see all the reason you should buy a "bike shop" bike.