because they don't. It may make searching for something harder, but it is usually safe to ignore items that are older, or on page2 of your search results. In addtion, part of itrader requires you to link to ther thread. And it lets you research past sales to get an idea of the average price of an item.
It doesn't make clutter because active items get bumped to the top and sold items fall to the bottom (except when a idiotic seller bumps his thread just to tell us it was sold