We haven't. We just can't produce enough carbon nanotubes cheaply to build a space elevator. Amongst other technical issues. One limiting factor of a rocket en

We haven't. We just can't produce enough carbon nanotubes cheaply to build a space elevator. Amongst other technical issues. One limiting factor of a rocket engine is the exhaust cone. This big dome-shaped piece of internally cooled structure tries to make the burning of fuel most efficient by controlling the shape of the exhaust reaction. In atmosphere you need a different size than in orbit to burn optimal, that's one reason why staging is done and the second stage is much different in cone size. Since in aerospike the direction and shape of the exhaust gasses is different, air pressure is used as a "dynamic cone" making single-stage to orbit" rockets much more feasable. I'm not sure if we want that at all, though.