He thought he could walk away. I suspect his legal team knew that was not happening and the best outcome was damage control and a relative slap on the wrist. The case is too big and public for him to walk.
It's worse than that. He probably could have pled guilty and cut a deal for 10 years of prison time or so. Maybe less, if he cooperated with prosecutors in getting some of the money back. But denying everything, going to trial, trying to evade all responsibility although he was in charge, and blaming everybody else does not go over well with juries.
SBF was never offered a deal. Why would the prosecution help him? They cut deals with his co-conspirators so that they could pin all the blame on SBF. The money is long gone.
All indications are that the DA offered no plea agreement, so the options were to plead guilty and hope to get the lower end of the sentencing range, or take it to trial.