Doesn't this apply when someone is actually charged with a crime?
I don't believe an indictment is the same thing.
I'm not really sure: but certainly the farther you get from an alleged crime, the more evidence is lost. If nothing else, potentially exonerating witnesses die, for instance. So once the prosecution has put its case together (which it has, through the grand jury process) the Sixth Amendment seems to me to require that the defense be allowed to begin preparing its case at once.
An indictment *is* a formal charge. It's a grand jury saying that there is sufficient reason to believe a crime has been committed to go to trial.