Because there is no cheaper solution that's reliable. I think when we figure out affordable global positioning without needing external infrastructure, we'll u

Because there is no cheaper solution that's reliable. I think when we figure out affordable global positioning without needing external infrastructure, we'll unlock a major technological milestone as a species. Right now you can do this for short periods of time with multi million dollar INS solutions which are used exclusively by the military. My guess is the answer will be in SLAM augmented with some other sources of data. See googles project tango.