Because Academies pick their own auditors and fire the ones who bark
No, because external auditors are poor at detecting fraud. That applies for external auditors in the public sector as well. External auditors are not good at detecting fraud. Not suggesting that auditors are rubbish or audits are pointless, but the evidence is that fraudsters are generally competent enough to hide their activities from auditors.
By the way, the auditors for a charity are appointed by the members of the charity, not the staff. Unless the majority of the members are participating in any fraud they are not going to fire the auditors for detecting it. And, of course, if the majority of the members are participating in fraud they won't be in a position to fire the auditors once the boys in blue get involved.
Do you have a link for that?
The estimate comes from the University of Portsmouth Centre for Counter Fraud Studies - you can see their report here. It is based on research the government used to carry out up until 2013.
They don't appear to have broken out education separately. I presume the figures for education fall under central government expenditure fraud and/or local government fraud. If we assume that fraud in the education sector is at the same level as for the rest of the public sector, it would be running at about £2.6 billion per annum on these estimates.