Consultants don't advise, they find 'evidence' to justify policy positions. They're also not subject to FOI requests and don't have to follow the same codes that public servants do. Governments love consultants because they usually don't actually want good evidence-based advice, they just want to tick the box that says they did 'research'.
I say this as a phd student who wants to go into academia but will probably end up working for one of these consultants because the government would rather give them money than fund actual research.