Are the departments you listed not operating in this way because of the budgetary constraints imposed by various governments through the years? Hiring temporary employees provides a lot of flexibility. If you have to deal with a 5% cut in your budget every year, for example, you just have to lay off your temporary employees. But if your staff is made up 90%, 92% or 95% of permanent employees, it will be much harder to deal with budgetary constraints.
Are departments working this way because of the budgetary constraints imposed by successive governments?