Currently the main thing preventing this is that GEDCOMs are limited. They are limited in size, and also in how far back in time they go. Profiles of people born more than 300 years ago are stripped out in processing.
Also, the change to requiring people to sign the honour code before uploading a GEDCOM has weeded out a certain number of dump-your-gedcom-and-go type people, who wouldn't bother doing merges and whatnot.
Coming very soon (last I heard, coming this month) is GEDMatches tool, which will actually search for matches between a GEDCOM and WikiTree before the profiles are even created. That way people will have the option to skip a branch in their tree, if they can see that the profiles already exist. We expect this tool to make a big difference!
One last thing to keep in mind: It is already so much better than it was. Anyone who's been active in the user groups for the last year or so (the first user group started just over a year ago) can tell you that. Mergin duplicates used to be one step forward two steps back. Now, a new one pops up now and then, but they're usually manually entered. It's now possible to keep up. :)