There's also a difference between a space probe, built once and having a mission that spans years or decades, and a rocket that gets built more often and operat

There's also a difference between a space probe, built once and having a mission that spans years or decades, and a rocket that gets built more often and operates only in Earth orbit (especially the first stage has to worry far less about radiation flipping bits). This isn't solely a difference between NASA and SpaceX, but also a difference in mission requirements.