The number of people in the labour force has change quite a lot over the centuries, and the number of people gainfully employed has tracked that quite closely. Ie unemployment has been relatively low and stable. (Despite eg women joining the labour force en mass over the course of the 20th century, or all the baby boomers, etc.)
That suggest the null hypothesis that to a first approximation the number of jobs is determined by the number of workers available. More workers seem to somehow lead to more jobs. (Immigration changes the number of workers, yes. But that's about it.)
