A certification body operating under ISO/IEC 17021-1 has to manage threats to its impartiality, and consultancy on a management system it then certifies is the clearest such threat there is. If we design your system, we cannot form an independent judgment about whether it works.
This is a genuine inconvenience for clients, particularly smaller organizations that would prefer to deal with one supplier. It is also what makes the certificate worth having. A buyer reading a certificate from the body that built the system learns nothing they could not have learned from the organization itself.
What we can do is tell you what the standard requires, explain why a finding was raised and what would resolve it, and deliver training on the standard itself. What we cannot do is write your procedures, conduct your internal audits, or act as your management representative.
Watch for arrangements built to look compliant: a consultancy and a certification body under common ownership, or a certification package that includes 'documentation support'. Accreditation bodies examine these closely, and a certificate obtained through one carries less weight than it appears to.