Because the fact that the internet has been around for a longtime the demand for good domains is high. Most people have resulted in getting domains such as http://www.online-games.com . Notice the – in the middle. There are many people who make a good living selling domains that they were lucky enough to register. The English dictionary is huge so there are always domains that were not thought of, but are still good. This requires you to spend hours at a domain registry site trying to find good keyword domains and then reselling them for 2X to 10X the times cost. There are other ways to get high demanded domains too. Misspelled domains are worth a lot as well. For example: imageshack.us is one of the biggest image hosting sites in the world. It is massive and gets 50,000 a week using the site. Since it's so big there's no doubt that many people type the url in manually in their browser. I there's no doubt that some make a spelling mistake.

Iimageshack.us would be the most common spelling mistake for this site which means that if you were to buy it for $6.99 you would be getting good traffic from it and can resell it for maybe as far as $600.