Because of stuff like this:

However, 4,000-5,000 temporary workers at GM would be hired in as full-time workers and paid at the full, tier-one wage-and-compensation rate that is estimated at $70-$75 an hour once wages, benefits and pensions are included. To make room for new hires, GM would again offer substantial early-retirement buyouts for workers.

Getting these temporary workers hired as full-time GM employees at the compensation rate was a top priority for the UAW, which has added few members at the Big Three and was seen as a graying union.

And this, and this, and lets not forget this.