By using off the shelf hardware and a system already in place, you don't run up the huge costs of designing and developing a totally new system. By getting to the Moon you get to the fuel. By mining the fuel you don't have to lift it to orbit. Slowly a transportation network is developed. In time you have fuel to sell.
Yes, and you can save a lot of money building a house by skipping the foundation.
Current expendables+shuttle amounts to a very poor foundation to build on. It is expensive, incapable, and likely to suddenly go away when you need it most. (Oops, there goes another shuttle up in smoke, hang on for two or three years while we get over our collective panic attack.)
For forty years we have been going into space without making getting to orbit any cheaper, more reliable or safer. You can't build castles in the air.
NASA and the space industry have come to the realization that they can't build the needed launcher, so they propose going ahead anyway.