Why are bike shop bikes more expensive than department store bikes?
Bikes come in many sizes, types, and uses. A department store bike is made to fit people of different heights, weights, and body types; plus it must wring out every dollar of profit and have an attractive price tag. They do this by compromising on components and quality. The frames are not well sized to an individual rider which can cause handling problems and limit performance. The parts are not the best quality. The store employees may or may not know how to build the bike when it arrives disassembled in a box.
Have we owned department store bikes? Yes. They are good for riding around the neighborhoods and park trails. They aren't the best for tricks, racing, cross country, or downhill. They aren't supposed to do any of that per the warning stickers anyway.
Bike shop bikes are generally more expensive because the frames come in sizes which match better body types, the components are higher quality, and they're designed for specific cycling disciplines. Bike shop bikes also tend to use newer technologies. It will be assembled by a bike mechanic, and you'll receive it tuned and ready. It is quality over quantity.
Can a department store bike be improved? Yes, you can sometimes replace the components with higher quality ones. The cost to upgrade the parts will exceed the bike's value though. That doesn't mean it is a garbage bike; it just means it isn't worth putting a lot of money into it to upgrade. A bike that retails for $150 at a department store needs tires, tubes, grips, seat, new derailleur, and shifter and brake cables. Not even halfway through that list we've hit $100 for just parts.
A big part of bike longevity is how it is taken care of. A department store bike can last a bit longer if it is parked in the garage, cleaned, and lubricated every so often. We aren't trashing hard on department store bikes as they have a purpose in the cycling world.
Ask yourself this: What are your bicycle needs? We can help you find what you're looking for.