Disclaimer: I'm a so-called LEGO Ambassador, i.e. I represent my LUG (Lego User Group) to LEGO, but I'm not a representative and/or work for LEGO itself.
LEGO is very interested in this 3D printing topic and had a workgroup on this on the Ambassador forum. I did not participate in this workgroup, but I can give some of the results. None of them come as a surprise, if one thinks this topic over, though, so I'm not telling any secrets.
- for standard bricks, it is too expensive, and except for a few classical basic bricks, there are patent and copyright issues.
- for bricks that do not exist from LEGO, this may work, but color, clutch power, surface structure, and durability are nearly impossible to match with current technologies
- best use for 3D printed stuff is to technically link LEGO parts to other things, e.g. a RasPi case that can be connected to a LEGO technik frame, where color and surface structures don't matter at all, and clutch power does not matter that much
- A lot of 3D stuff is accessories for Minifigs, like tools, weapons, hair pieces, etc.
Basically, while there are thousands of 3D data sets for LEGO parts available on the net, actually printing a box of bricks to build is far from being practical.
LEGO uses 3D printing in their design process, but only for prototyping. They are more likely to cut and glue existing parts for the prototyping, though, as this is still faster and better.