Because wage slavery is ultimately not much better than slavery, and its abolishment does not necessitate the collapse of society. A world where no one is forced to work is a better world.
Wage slavery is just fine as long as I can feed myself. Time has proven that the fantasy that we will all kick back and let the machines do all of our work is a fallacy.
Just because it hasn't happened yet, doesn't mean it won't happen in the future.