Because the beginning growth a muscle is the easy part, that is why we tell people to stay natural as many years as possible, because growing from the point you are now is slow. Specially if you are not giving bodybuilding 100 percent, if you are you will constantly keep growing, but slowly. You will be mixing things up, working out what your body likes and dislikes. You can't do a steroid cycle and turn into the hulk, you may never if your genetics don't allow for it, regardless how much gear you use.
"Whether You Think You Can or Can't, You're Right"--Henry Ford