One reason for this (it might be suggested) is the time constraints of the summary jurisdiction. I don't think that's why. Magistrates often go to great lengths to learn more about an offender when deciding their sentence. More likely the reason it's rarely done is because it's rarely done; what was a habit has now become a tradition, verging on a rule.
(Another reason might be that with the lower number of cases and more serious offending involved in the higher courts, the prosecution is more prepared to challenge assertions favourable to an offender.)
It's true that preparing a witness to give evidence is time-consuming, and sometimes difficult. But oral evidence can frequently be more effective than even the most well-crafted written reference. It's the human touch.
Nonetheless, it’s usual — especially in summary courts — for assertions by an accused to be received without insistence on formal proof.
Authority for this practice is found in R v Storey  1 VR 359 at 371 – 2:
We have spoken of ‘proof’. Ordinarily, much of what is relied on in sentencing is not the subject of evidence given on the plea. Judges have always relied heavily on what is asserted from the bar table and we see no reason why that practice should not continue. We are not to be taken as suggesting any departure from current practices on sentencing hearings. As we have said, judges can, and commonly do, act in such hearings on matters that are not proved by evidence that would be admissible at trial. There will, however, be cases, we venture to suggest relatively few cases, in which there will be significant disputes of fact that can be resolved only by the ca11ing of appropriate evidence. But that is to stop well short of adopting the procedure by way of so-called ‘Newton hearing’ that has been adopted in England and Wales. The procedures for hearing pleas in this State have not been shown to be wanting, and while we readily accept that “Nothing gives a bigger sense of injustice to a convicted man than false statements being made about him after the verdict” we do not accept that present procedures are deficient in this respect: R v Storey  1 VR 359 at 371 – 2.
So, what happens in those cases when there are disputes of fact at sentence?
It was settled in R v Storey, and is now given statutory force at s 141 of the Evidence Act 2008, that the standard of proof for an accused in a criminal hearing (including on a plea, if the rules of evidence have been imposed) is on the balance of probabilities.
When the prosecution objects to or disputes assertions favourable to the accused, they need to be proved by admissible evidence.
What I have said is not to deny that the sentencing judge should be fully informed, or that desirable practices as they have developed should not continue. In practice sentencing proceedings are conducted with a degree of informality. Unnecessary insistence on the strict rules of evidence is in no one's interests in sentencing proceedings, and the customary co-operation between the Crown and the offender and making of admissions by the offender should so far as possible be insisted upon. But if there is good reason for objection to evidence in sentencing proceedings the objection when taken must be resolved and, apart from statute, must be resolved by application of the rules of evidence. In the absence of a direction pursuant to s 4 of the Evidence Act, the law of evidence unaffected by that Act applies: Bourchas (2002) 133 A Crim R 413 at .
The typical formulation of this is that when a sentencer proposes to take into account matters favourable to an offender, those matters need only be proved on the balance of probabilities; when the sentencer wishes to take in account matters adverse to the offender, they need be proved beyond reasonable doubt.
But the court is not bound to accept the view of events most favourable to the accused unless they can be disproven by the prosecution. For example, in a drug conspiracy case it was put at the sentence hearing that the offenders had abandoned their conspiracy, based on answers in their record of interview. The Victorian Court of Appeal gave that submission short shrift.
If the applicants had been serious in their contention that the project had been abandoned they could have given sworn evidence to that effect before the sentencing judge. It is remarkable how seldom accused persons are prepared to go into the witness box to lend verisimilitude to the sometimes bald and unconvincing assertions which counsel make on their behalf: R v Raptis, Lilimbakis and Sinclair (1988) 36 A Crim R 362 at 366 per Young CJ, O'Bryan and Tadgell JJ.
A lawyer is, on occasion, given instructions that strain credulity. One wonders if those instructions would be pressed so firmly by the client if it was the client, and not their advocate, who would have to explain their story to the court. But then, that's the raison d'être for advocates: to act as a buffer between their client and the Bench, and to articulate those matters the client might not be able to adequately articulate themself.
But it's not all one way. Occasionally, prosecutors will insist that a Prosecution Opening (or in the Magistrates' Court, the police summary) represents the One True Version of the Truth, and that if the offender wants to put anything different, they must give or call evidence.
This occurred in Ashton v The Queen  VSCA 329 at  – . Counsel for the offender told the Court the offender disputed one of the facts contained in the Prosecution Opening. She submitted that if the disputed fact were accepted it was an aggravating feature, and hence for the prosecution to prove beyond a reasonable doubt.
The sentencing judge said ‘... if you want to assert something that is to the benefit of your client apparently and is inconsistent with the Crown case you are going to have to call evidence on it’ (quoted at  of the judgment).
After further discussion, the prosecutor stated the Crown intended to proceed on the Opening provided, and submitted that the offender ought to give evidence if he wanted to put anything different.
On appeal, the Court of Appeal held this was not the proper way to use a Prosecution Opening.
 ...The critical issue is that the judge accepted the disputed fact as established, and relied on the fact as a matter adverse to the appellant. The difficulty is that, as counsel made plain from the outset, the fact was disputed. In those circumstances, if the Crown wished to persist with that allegation of fact, it was for the Crown to call evidence sufficient to satisfy the sentencing judge of the fact beyond reasonable doubt.
 ...In my view, the judge erred by approaching the matter in that way. The starting point is that, as conventionally occurs, the Crown opening constitutes an agreed factual basis upon which a judge passes sentence: R v LJF  VSCA 134 at . However, in this case a fact in the opening being disputed, it was for the Crown to call the victims to give evidence in substantiation of the fact, which effectively meant providing the appellant’s counsel with the opportunity to cross-examine them. By taking the course that he did, the judge denied the appellant that opportunity, and acted upon facts that were not before him.