1) Why is the labor supply so low? Workers are not interested in the relatively low salary for relatively bad working conditions.
2) Why do salaries not rise? Mid-term the hard cap for salary increases is the increase in worker productivity.
3) Why does worker productivity not rise? It does rise, but far less than inflation (about 1% p.a. for the last ten years). Mostly due to micro-optimizations of existing jobs.
4) Why can't we increase productivity significantly? It would require a shift in how we structure work, i.e. stop doing things that shouldn't be done in the first place instead of optimizing them. Existing jobs will barely change, new ones will.
5) Why is this process not faster? Because both institutions and people are unwilling to change behavior on a large scale. Institutions because they did not need to (before the great resignation) and worker mostly out of habit and complete lack of economic understanding, aka. salary must be earned in form of generating revenue, not in the form of passing time.