I'm by no means an expert diver. I do have a technical background in physics. I'm fairly certain with the right tools the right books and enough time I could build every part of a dive system from the tank to a dive computer. Even a re-breather system.
Unfortunately I don't have the time, and fortunately many others have done that work already. So I simply find it most practical to rely on the experiences and work of others for a small fee rather than spend the time, effort, and risk doing those same things myself.
So while I think most anyone could potentially learn everything themselves in all aspects of the sport, it's just easier and safer to learn most things from someone who's done what you'd like to do a few times before. Pretty much exactly what Teamcasa said.
Unfortunately I don't have the time, and fortunately many others have done that work already. So I simply find it most practical to rely on the experiences and work of others for a small fee rather than spend the time, effort, and risk doing those same things myself.
So while I think most anyone could potentially learn everything themselves in all aspects of the sport, it's just easier and safer to learn most things from someone who's done what you'd like to do a few times before. Pretty much exactly what Teamcasa said.