Imagine this: you have no car, limiting your ability to travel to a job. You have only a high school degree, limiting your prospects. You have a child, limiting the times you can work. You have very few marketable skills, limiting the specificity of a possible job.
Do you think you'd have a whole lot of ability to be picky?
perfectly explained. By the time most people realize all the happenstances (er... personal requirements) necessary to be picky about the work they will perform, they have already made the decisions that have defined their potential work options.
There are the rare few that select service work over better paying work (CS/Engineering grads choosing to serve, bar-tend, barrista, etc. This is an accepted identity among people in Austin, TX and I'm sure other areas). Some of this may be under-performance, but I think it is more a respect for simplicity and a desire to appreciate life and people: they do it because the want to.
More often than not, service work is filled by people with a need for any job at all.
Do you think you'd have a whole lot of ability to be picky?