I think a lot of 3D printer hobbyists turned to 3D printing once it got good enough because we don't have space or easy access to even cut things or have a stash of parts. You're right, I can get a M6 x 1200mm stainless steel rod for AUD$5. But then to cut it accurately would need a few more tools. So you kinda try and limit things to what you have. And you can do quite a few useful things with just 3D printed piece and the odd screw here and there.