Because they want money they are not entitled to. uhhh extortion, already covered.
So every group of employees that bands together is trying to extort an innocent, helpless employer? This is an extremely narrow and incorrect view.
The UAW of the 1960s and 1970s did a horrible job of protecting its members and promoting itself. I am ashamed to admit that my father was and is a member of local 569 (at one time the worlds largest local).
However, I am quite proud of the fact that my great-grandfather participated in the Great Sit-Down Strike in 1936. It needed to be done.
The fact of the matter is this: Very few employers pay their workforce what it's worth. They pay the workforce what they have to to retain them.