That's really because Congress doesn't want them to do any more than verify the claims in the application and see if it meets criteria for ultility and novelty. Then new patents get published so the assumption is another prior holder will challenge it at that point.

The system really needs fixed but there are so many interest groups that oppose changes. Right now patent examiners can make a nice bonus (like 10-20% of their salary) for beating their production quota. Also they can be fired (at least in their first few years of employment) for failing to meet the numbers. So everyone is concerned about getting patents granted as soon as possible and not upon whether its a good patent or not.