Sam Bankman-Fried's sentencing reflects the fact that a pre-sentencing report has to be prepared and a sentencing hearing has to be prepared for. These tasks aren't done or scheduled before there is a conviction, because they might not be necessary at all, and might be different if he was convicted on some rather than all charges. But since sentencing hearings aren't scheduled prior to a conviction, the sentencing hearing has to be set for when the judge and the lawyers are available, and their schedules fill up with other things in the meantime.
Keep in mind that in criminal cases, judges have immense sentencing discretion, much of which depends upon factors not presented in the trial on guilt or innocence. A criminal defense lawyer's job is much more focused on sentencing and much less focused on the guilt or innocence phase, in most cases, than the general public realizes.