The first thing one must keep in mind with graded cards is that it can cost quite a bit to even have a card graded in the first place. PSA grading costs (if I recall correctly) ~$25 USD per card. When a seller sends a card in to be graded her or she is taking a gamble, because only the most highly graded cards will be worth the extra fee.
When you buy a graded card, you are not only getting the card itself, but also a professional guarantee that it is as described by the respective organization's grading scale. A card graded 9 or 10 will be absolutely perfect, with virtually no damage whatsoever. And these professional graders are strict. Even if you could only see a dent in the narrowest angle of light they will dock points for it. You are paying for the service, the guarantee, and the card. Oh, and you also get a nice display casing to keep it in the condition labeled.
When it comes down to it, very few people are so picky that they would spend sometimes 5 times as much for a graded card. But those people will go to any length to ensure what they have is as close to perfect as they can get. To them it's worth it.