Since NASA can build the rockets more cheaply
Where do you get that crazy idea? $9 billion spent on the Ares 1 and not even a rocket launched, except the Ares 1-X which was just a 4-segment shuttle solid rocket booster from ATK with a fake 5th segment on top and a dummy payload -- a $500 million fraud. For the same amount SpaceX designed, built and tested 2 rocket designs, upgraded a launch pad and is designing building and testing a capsule for cargo and crew. The ignorant will say they did it with government money on the COTS contract, but that money is paid out only on performance, and the COTS launches are yet to happen -- most of the money spent so far by SpaceX is their own money. Furthermore, they have billions in contracts outside of NASA, making NASA's contribution to their revenue less than 50%. Plus NASA's plans is to do R&D on fuel depots and on-orbit assembly, making heavy lift like the proposed Ares V unnecessary.
If you ignore the fact that some large fraction of NASA is really driven by the military
They prefer to call it piloting thank you very much.