Because there's essentially a whole second sentencing trial. After the verdict, the Parole Office prepares a Presentence Report (I'm not sure if these are public? I'm always looking for them and never finding them on PACER†), the "PSR", which is like the opening bid for the recommended offense levels and grouping. The prosecution and defense then respond to the PSR with exculpatory, mitigating, or aggravating claims, victim impact testimony, your scout leader when you made Eagle Scout, all that stuff. Only then does the judge work out what the actual sentence is.
The guidelines calculations, contrary to public belief, are pretty easy! You will have a pretty good success rate just carefully reading the charged statutes in the indictment and then following the instructions in the guidelines. But the PSR, prosecution, and defense filings all take a bunch of time, and involve "reporting" or "research" or whatever you'd want to call it.
That process ostensibly can't even start until everyone knows what the precise convictions were.
For what it's worth, I think there's a very good chance he just gets the statutory maximum penalties here, because the dollar loss and victim count numbers max out the 2B1.1 guidelines (which, for instance, cap out at $10MM in dollar losses). Attributing specific dollar losses and victim counts is ostensibly one of the things that makes sentencing take so long, but as long as it takes I think the outcome is a foregone conclusion.
† Later: they're definitely not public; there's a whole victims rights thing about getting victims partial access to PSRs so they can be heard in the sentencing process. I guess I can stop looking for them.