Why is no reference made to the information used by the internal audit department and/or other external auditors (such as a service organisation that the client uses) in the auditor’s report?
Short answer – because the audit opinion is the auditor’s alone.
To refer to “others” – internal audit, management’s expert, auditor’s expert – or the work of others – might suggest to the reader of the auditor’s report that the auditor’s responsibility is somehow shared with others or that others are in someway responsible for some of the information in the financial statements. So ISAs prohibit this.