the thing is that NASA did indeed try to build reusable spaceships themselves back in 1968 with the space shuttle. the thing is: they didn't make it. their space shuttle sucked and cost more than non-reusable rockets. Then some 30 years later SpaceX came along and somehow did the impossible, which is to build reusable rockets that are actually cheaper than the previous, non-reusable rockets. So, that's a big step forward. As a consequence, NASA started paying SpaceX for the fine product they were making that they tried to make themselves previously but failed.