As well as trust, there is perhaps also a bit more safety in using a consultant. In theory, the consultancy is offering a guarantee of the consultant's skills and can replace them should they prove unsuitable or if they stop being available. This also gives greater continuity when people are moving between companies/roles/contracts.

Outside the world of discrete software products, technical architecture may also be regarded as an ongoing role (unlike in the building industry), and is slightly less suited to the contractor/consultant market where it could be costly if undertaken by retainer without a continual stream of projects.

No doubt there are examples of where businesses have processes in place to prevent islands of knowledge forming and can work successfully with permies, consultants and contractors alike.