As others have pointed out: Compare the budgets.
That “first success” was actually on the back of a long series of related rockets with technology and engines inherited from a huge missile program. Those NASA eggheads didn’t start from zero on a shoestring budget and make things work on the first try! The Saturn V was just a stretched version of the Saturn series of rockets. These all cost hundreds of billions in today’s money to develop!
Second, they’re not “the same thing”. A single-use piece of technology has very different design constraints and engineering considerations as a reusable piece of technology.
A single-use weapon is a bomb. A reusable weapon is a sword. Just because you can shove a fuse into some explosives doesn’t mean you can forge a sword that won’t shatter on first use.
An equivalent example from space technology are explosive bolts. NASA uses them extensively, SpaceX never does… because they’re not reusable and not up-front testable. They’re expensive too. So instead they iterated (and iterated!) on vacuum-rated actuators that can serve the same role. This is a non-trivial exercise that resulted in a few RUDs. This is why NASA didn’t even try! It’s harder and not needed if reusability was a non-goal.