Because:
1. The NG program does not contemplate major iterations like falcon had (from 1.0 to landing legs to FT...and everything in-between). Whether or not one agrees with the philosophy, near as makes no difference Blue is planning on full success on their first launch. That's one (of many) reasons why its taking so long--there's so many critical pieces that have to come together. (Make no mistake, they are coming together, and some of the pieces are quite impressive...they just take a traditional while and Blue is notoriously tight lipped about progress in the public domain so nobody really knows what's going on)
2. They've demonstrated they know how to land a rocket; figuring out what kind of thing NG should land on is an incremental step. They're so close that its pretty stupid to throw away a stick without even trying to land and reuse it.