The Starter Bike
Starter bikes can fall into a number of categories. Some start with an old bike that a friend or family member gives them, then buys a nicer one after figuring out what kind they actually want (or after the one they've been given falls apart). Others will buy a second-hand bicycle and will later upgrade in the same manner.
Then there is the "budget bike." New cyclists are often reluctant to spend too much money on a new bike, and $500 seems to be a typical budget for those just starting out. So they will purchase a new bike in this price range, but will soon be dissatisfied with the quality. They will then replace it with a higher quality make and model later.
Some bicycles end up replaced because they are the wrong kind of bike for the cyclist. A novice might love the idea of the Dutch Omafiets, but will discover that they just can't ride it in their hilly town. Alternatively, a beginner may start out commuting on a roadbike based on a bike shop's recommendation, but will find it uncomfortable.
Even among seemingly similar bicycles there are differences in handling that may not be apparent at the time of purchase. A perfectly good bicycle of brand X is often sold and replaced with an equally good (and visually similar!) bicycle of Brand Y after just several months of ownership, because the latter better suits the cyclist's ride quality preferences.
All in all, I would say the majority of American transportation cyclists I've spoken to who have been at it for as little as a year, are not riding the same bike now as they did when they first started. And to me this is kind of interesting. Does it mean that novice cyclists would benefit from educating themselves better before making the initial purchase? At one point I thought so, but I no longer do. There are plenty of stories showing that you could be extremely well informed and still buy a bike that is ultimately not right for you.
Another conclusion that is tempting to draw, is that since chances are your first bike will be replaced within a year anyway, buy a cheap one while you hone your preferences. But one thing to consider here is that a low quality bike can discourage cycling entirely. Typical scenario: things start to rattle or go wrong with the bike, and a novice just stops riding. Or, the bike feels extremely uncomfortable or inefficient, and the cyclist decides that this is characteristic of bikes in general. A higher quality bike, even if it is not right for the person in the long run, is more likely to inspire a lasting interest in cycling.
Finally, there is the simple fact that our preferences change over time. Sometimes this has to do with experience. There are bicycles that feel great to a beginner, because they are exceptionally stable and easy to control for those who have not been on a bike in a while. But as the cyclist grows more confident and picks up bike handling skills, the same bicycle can begin to feel limiting due to a lack in speed or maneuverability. But changing preferences need not be a matter of skill - sometimes cyclists just feel like a change from what they were initially attracted to. It seems that with transportation bicycles, there is really no way to know where on the spectrum you will settle. The Starter Bike phenomenon may just be part of the course.