It is quite possible to do research as a hobby. It is very difficult to do proper research training as a hobby, and essentially impossible to do a research appr

It is quite possible to do research as a hobby. It is very difficult to do proper research training as a hobby, and essentially impossible to do a research apprenticeship (which is what a PhD is) outside of an environment where you are being supervised by other PhDs. If you have a spouse or close friend who's prepared to supervise you, fine - but otherwise it's likely to be at a university or research lab. Coupled to that, PhD slots are generally funded, so university at PhD level is normally less 'expensive' and more 'opportunity costs'.