My guess is that this hasn't changed due to union action or lawsuits because (1) teachers are salaried, and tend to be over the limit where overtime pay is legally required; and (2) union organizers don't think they can get any more money without giving something else up.
Also, in the states where this problem is the worst, there’s usually “right to work” legislation and other rules that do their best to defang collective bargaining power.
