The same reason a good power supply costs $100. Because it is well made from quality components. A chair supports your body and in my opinion should not be skimped on. Look for a used Steelcase LEAP chair on eBay. Very comfy, highly adjustable and overall a solid chair. More so than the over hyped Aeron, in my opinion. Only downside is the cost - about $400 - $500 delivered although there are some deals now and again. Make sure if you get one that it has armrests and is "fully featured".

That or build your own chair, like I did.