As the body of human knowledge grows, professionals in an area become increasingly specialized.
As humanity’s collective knowledge grows, it is no longer feasible to be an expert in all arenas. Doctors used to just be doctors. They would diagnose the sick, do surgeries, and perform therapies all from their one little office building. Now there aren’t really many “general practitioners” in hospitals - and general practitioners often just serve as the gateway to get the patient to the appropriate specialists. What used to be considered “specialized medicine” isn’t even that specialized any more. You’re not just a surgeon, you’re an oncologist, and not just an oncologist, but an oncologist who specialized into pediatric oncology. We have gone from generalists and specialists to specialists and ultra specialists. Not just in medicine, either. It used to be possible to be a Renaissance man and know literally everything there was to know about math, physics, astronomy, chemistry, and engineering. Now our chemical engineers have specialized into petroleum engineering versus biomedical engineering versus materials engineering.