A company may not want its CPAs (note this is equally applicable to CAs) to audit a client's records because the auditors gain a substantial amount of competitive information during an audit. However, CPAs are bound by confidentiality under their Code of Ethics for Professional Accountants. Also, a CPA's knowledge of the industry gained from having several clients in the same industry provides him or her with insights he/she may not have otherwise had.
Think about the challenges posed here to honour confidentiality (APES 110.140 (pre 2018) or 110.114 (post 2018)) when complying with obligations for changes in professional appointments (APES 110.210 (pre 2018) or 110.328 (post 2018)).
I hope by now, you are beginning to develop a sense of the challenges of ethics and quality control and the key role of professional judgement in such decisions … not easy to have specific “rules” in place is it?