Each additional step requires solving a lot of new engineering problems and usually some new physics too, and it's not a linear process: many techniques are proposed that become cost effective only when previous techniques hit their size limits, and then a lot of research is done and most techniques are never gotten to work at scale and just a few pan out and become the next process.

This is a great talk on the history of chip processes: https://www.youtube.com/watch?v=NGFhc8R_uO4

This is about some of the challenges that had to be solved for a current modern process: https://www.youtube.com/watch?v=f0gMdGrVteI