Companies can fairly easily break a union. After call, why does Ford need a Detroit plant when they can use Mexico.
But governments are lead by politicians who get money from unions. Union supports leader, leader pays union, union supports leader. No incentive to keep costs down at all. It's actually better for politicians to have strong public unions.
Teachers unions make it pretty much impossible to fire someone. You pretty much need to molest a child in places with a union (after you make tenure).
Hence the US states' and cities' unfunded pension debt crisis. Only solution to it in democracies is for non government employees to actively participate in local elections.