There are 2 factors that combined ensure the public sector continues to grow and continues to have lower productivity and more redundancy than the market sector.

One is a position classification system rewarding higher points and thus higher classifications and in turn higher salaries to managers who can "creatively"  justify more staff and higher budgets for their organizations.  Doing less with more is a promotion strategy in the market sector but the road to oblivion in the bureaucracy.  So the public sector bureaucracies pay off on growth for growth's sake and are perfectly OK with marginal productivity.

The other factor is that for politicians it usually is a sound vote getting strategy to support new programs and handouts rather than to hold the line on public spending.  New programs and handouts are supported by people receiving said handouts, not the least of which are the greater public sector employees who reciprocate with votes, canvassing and 3rd party attack ads.