It's probably because when you copy what you see only you start to have problems with drawing from imagination.
For example the head; if you copy from photos only you'll have a hard time drawing the head from different angles (unless you have a photo for the exact angle that you want), but if you understand how the head is constructed (sphere, jaw, eye placement, etc) then you can draw the head from any angle you want. It's the same with figures as well. If you don't have a photo of the exact pose you want to draw then it's very hard to draw a figure unless you know the basic shapes the figure is made up of.
But drawing with your eyes is an important skill too. It's actually a skill that I need practice on myself.
I would suggest practicing construction. Take a photo of an animal and construct it using only basic shapes (cube , cylinder, sphere). And if that's too hard then maybe start with practicing drawing basic shapes from different angles.
I hope this helped.