Just a question...
If people get small fish and start out with a small thank, is this really a big problem?
I know it's a lot of extra work with setting up new tanks as the fish grow, but wouldn't this be possible, even though it's not the easiest way to do things?
There's no real reason to do that, though, and plans for bigger tanks almost always get delayed--counting on being able to buy a bigger aquarium in the future is a risky way of doing things.
In the case of goldfish (and many other species), you need a large tank right off the bat anyways because they produce so much waste. In a small aquarium, ammonia (and hopefully later nitrates instead, once it's cycled) will quickly build up and begin poisoning the fish. Starting small with goldfish would mean setting them up in something like a 20-gallon with plans to upgrade--not a 6-gallon.
You also run the risk of stunted growth. Many fish produce pheromones that stunt the growth of their tankmates. This isn't normally a problem in a decent-sized, well-maintained tank. But in the confines of a "desktop" type aquarium, you can end up with stunted fish (leading to other health problems, as you might imagine).
I just don't see a reason to purposely house fish in an aquarium that will need to be immediately upgraded in a month or two anyways. With some species, there's no problem as long as the tank really is upgraded on time. But that doesn't seem to happen in most cases, sadly. And when it comes to large, active, messy fish (like goldfish) there's no reason at all to start them out in a small aquarium...unless "small" means something like a 20-gallon or 29-gallon to hold them while a pond is built