Margaret Hamilton, NASA's lead developer for Apollo program, stands next to all the code she wrote by hand that took humanity to the moon in 1969
Not a whole lot of things cost a billion dollars to attempt again. We could put it all together again eventually but the amount of R&D needed to return to where we were just hasn't been worth the benefit. We have countless unmanned craft all over because getting scientific data doesn't require a dude to be there anymore.