Unions are incentivized to bargain for as much compensation as possible and politicians are incentivized to give it to them if union workers make up a significant chunk of votes. Politicians don't have to be concerned with whether or not those obligations are realistic because they'll be out of office by the time the whole thing falls apart.
Yes, they are. And companies are incentivized to bargain for as much profit and as little employee compensation as possible, yet people here seem to consider that a moral imperative. I cannot, and will not begrudge unions for trying to get the best deal for their members while that is the case.
Edit: I think I should note that I'm specifically referring to unionized government workers.
Not necessarily, especially when employees are in high demand. Netflix has done quite well by paying employees well above average in order to effectively have their pick of the talent.
Paying employees poorly can increase training costs, recruiting costs, cause negative customer perception, etc.