The work of rationalising / downsizing / redesigning / cost reducing large organisations is often undertaken by external consultants as, in theoy at least, they have no vested interest in the outcome. They didn't work in the organisation in the past and won't work there in the future.
The people you mention, "accountants, economist, tax consultants managers etc" IME are generally amongst the least well qualified to run a programme of change management, business transformation and organisation design. Some of their skills and input will be required during the project, but only as contributors, not as project managers.
The skills required can be found in places like for example, Sheppard-Moscow [broken link removed] (no connection, just an example)