I mean, it isn't like there are not multiple precedents for NASA to find a surprise safety issue, talk it down, and then see it literally blow up in their faces.

NASA is an institution and the incentives align with launching despite risk in cases where the risk was completely unanticipated. The project has its own momentum that it has gathered over time as it rolls down collecting opportunity costs and people tie themselves to it. If you think an astronaut would pull out of a launch because of a 5% risk of catastrophe... well you are talking about a group of people which originated from test pilot programs post-WWII where chances of blowing up with the gear was much much higher, so even though modern astronauts don't have the same direct experience, it isn't beyond reason to assume they inherent at least a bit of that bravado.