Because they are treated abominably. They aren't paid a living wage. They have shitty benefits if any, little hope for a decent retirement, and can't even get reliable hours in which to have a life. Most have no job security. This is ruinous to human health and happiness, and to the stability of society at large.
Unions improve all this and more. They don't just help union members, but everyone. Remember:
8 hours for work
8 hours for rest
8 hours for what we will
The people who brought you the weekend! \o/
There is another option, though: cooperatives. In this model, unions are unneeded because the workers and owners are the same people. The "nonprofit" descriptor is sometimes true, other times misleading -- if a coop does particularly well, it winds up with extra money that can either be invested in capital improvements or paid out to members, depending on structural details.