A lawsuit case like this is a civil matter - judgements / settlements are made based on some violation of a legal agreement between two parties. Violation of that legal agreement may or may not be a criminal issue.

For someone to go to prison, a criminal case would be needed, requiring criminal acts - not just violations of a legal agreement - and government prosecutor willing to bring charges against the parties allegedly responsible.

It seems there were (probably) criminal acts here, but as of yet we've not seen a prosecutor go after the case. These kind of things can be further complicated in the US because of the different standards of proof required for each case type. In a criminal case, the state must be able to prove "beyond any doubt" that the defendant is indeed guilty. Civil cases have a much less ironclad standard for guilt/responsibility, and can often be easier to "win" as a result.

Its certainly possible that government prosecutors are aware of the issue, and aware of the civil settlement, but don't believe they could prepare a compelling enough case based on the evidence available to win a criminal trial against undoubtedly well-legally-represented executives.

No one in the US goes to prison except low level drug offenders.

Everyone else is too big to jail.