At any given moment there are a fixed number of jobs, and so a fixed number of "good jobs". I can grant that the number of good jobs isn't fixed on the scale of years. But people that want/need good jobs usually can't wait years for them to materialize. On the timescales that matter to individuals, the number of good jobs is fixed and so the competition for them is effectively zero-sum.

There also seems to be a scale issue when it comes to the availability of good jobs. That is, not all jobs can be good (i.e. high paying) and there is an essential order-of-magnitude difference between the number of high paying jobs and the number of low paying jobs required to make the economics work. So the proportion of good jobs is effectively fixed even on longer timescales.