Well many of them, depending on which list you are seeing, are in a thirty day limbo. Both profile managers need to approve a merge for it to move forward, but are large number of duplicates are managed by people who are not currently active on WT. These will be automatically approved after a thirty day period if no action is taken.
The rest which still number in the hundreds are just a backlog. Merging takes time and sometimes requires some expertise. It seems that merges get completed about as fast as they are proposed (which is a credit to the efforts of many volunteers), but catching up seems unlikely. In my opinion the major challenge is that often completing one merge leads to proposing several more. Very quickly any effort to reduce the number will grow it, which is what should happen to maintain the health of the tree.