I am not going through the whole reason because it is all explained in a lot of posts in a lot of threads on the topic of hand coding versus Cam.
There are reasons:
One is that when I taught myself G-coding to move my business from totally manual production to CNC it was more efficient to do hand coding rather than spend months learning CAD/CAM. All the parts were designed and being made manually so hand coding was a snap.
Another reason came apparent when I had a guy who did do CAM; the programs run slower, sometimes a lot slower. We do small scale production anywhere from 10 to 2000 parts per batch and the batches repeat every three months or so. Even a few percent difference in cycle time can add up to a significant difference in total batch time.
Yet another reason is that when I design a new part I do not do it using CAD I do it on the machine and develop the production tooling at the same time. This way I make the initial protoypes on a simplified version of the fixturing that will be used for the production volumes and if I have to tweak features to enhance the efficiency of fixturing I can.
If you are only doing one-offs and if you take in outside work that has been designed by some nit who does not have a clue how things are made you are probably better off. indeed you probably have no choice, to use CAM. My situation is not common.