Sacking someone for making a mistake[1] is often more about petty revenge than any desire to improve things. It's a stupid policy. Someone who's made an error and learned from it is less likely to make it again than someone who hasn't made the error in the first place. This is the exact reason why experienced people are better qualified to do jobs than new people. Experience is valuable.

[1] By that I mean an actual, hard-to-foresee mistake that no one else caught rather than just incompetence or lack of ability. It's safe to assume that people without ability don't get in to decision making positions at NASA though.