Why do teachers have to strike?

Now I have to admit that I have a serious bias against teacher strikes in general. They hurt no one more than they hurt the kids. I was the victom of several teacher strikes when I was a student and I will never forget those lost days.
But at the same time we see situations all over the country where local governments seem to want to balance the budget by cutting back on teachers or teacher compensation. One hears people talk about "teachers are underpaid" or "teachers are more important than sports stars" but seldom does anyone jump up and say "hey, let's raise some taxes and pay teachers what they are worth!"
I found a couple of links over at the Education Wonks today. One is to news that teachers in Detroit are talking about striking. The other is that teachers in Gary, IN are already on strike.
The Education Wonk asks if teachers should be allowed to strike. And that is a good question given how much a strike hurts kids so much more than it hurts the political leadership. But I think a larger question is "why does it ever come to a strike?" If our society really values education and really values teachers why would conditions ever get so bad that teachers, among the most dedicated of professionals, feel that they have to strike to get what they believe is fair and reasonable treatment?