> How do you explain so many people believing it is safe?

The article itself answers this question: institutional incentives leading to heavy social pressure to agree with the groupthink and declare something is safe when it is not. And we know that the scenario it lays out is highly possible, because it has already destroyed two Space Shuttles. Now that this has happened twice, the burden of proof is on the people saying it's not happening again, especially when the OIG's report directly contradicted what NASA had been saying about the heat shield up to that point (indicating they were lying and had to hastily retcon their story).