The notion behind the Biblical quote of reaping what one sows can immediately apply to the migrant workers because they do not seek to establish roots and permanent connections. In moving from one place to another and being rootless both in physical and emotional senses, the migrant worker reaps what is sown. They embrace a life of detachment from others and the result is that they lack attachment to other people. What they reap is what they sow. The distinct way in which Steinbeck describes the living quarters at the start of chapter 2 in Of Mice and Men helps to enhance this.
With this in mind, I think that it might be a bit harsh to suggest that migrant workers are deliberate in their approach to life and its loneliness. Steinbeck offers a vision in which the lack of work and need for material sustenance compels them to embrace a life that no one would really want. Throughout Of Mice and Men, Steinbeck shows little joy in being a "bindle stiff." It is for this reason that it might be more accurate to suggest that the migrant worker reaps what others have sown for them.