Drawing synthetically or constructively is a different skill. And learning to draw observationally, that is learning to see, is still a prerequisite.
Knowing anatomy, by the way, doesn't necessarily help you draw. I've had doctors that specialize in anatomy in class and they were terrible at drawing until they learned to see. They could recognize and label the parts, and they could even do a simple 2D schematic type diagram of muscles, but put a real 3D, foreshortened person in front of them and they were just as awkward as anyone else.
Well, when you're drawing from a long pose (week or longer), the model is bound to change her/his shapes slightly.. So just drawing what you see is not enough, you have to understand the 3-dimensional structure of the thing you're drawing at least on a basic level.
But also, I agree -- foreshortening is hard until you learn 2-point perspective and can "see" perspective points that you can easily build up the foreshortened feature from.
Seeing isn't necessarily a 2D process; and not just because our vision is binocular. Our visual system is very complex. The hardware is, optically, very crappy. Our image processing and integration software is very complex, much more complex than we really are able to understand at this time.
And it behaves almost nothing like a camera. Even mechanically, the optical projection is very different than a camera makes, or what one would get from a linear perspective projection.
There is a projection, but the "image" is actually composed of multiple viewpoints over time. The adjustments an artist must make for small changes in the model's pose is nothing compared to the varying nature of the mental image you actually "see" inside your brain. It is far from static, and is in fact very dynamic.
Which is why that drawing from life is considered much more important than drawing from a photograph when learning to see.
Note that I'm not knocking the study of structure or synthetic processes like linear perspective. They are important tools and aids in perception and image synthesis. But a good drawing, or painting, or photograph for that matter, is not a mere 2D projection of light and dark. It is an encoded record of a multitude of perceptions, or perception artifacts that your brain then interprets, inside it's magic "seeing" box, which then produces a simulacrum of a perceived event or process.
Else making a good drawing or photograph would be an entirely mechanical process that anyone could make by following a few simple rules. Instead they are like moments of insight into a scene or event.
And all of this is relevant if you are creating software that is going to communicate visually. When you are building your data structures and models inside your head, it is a very different from that which the experience the user has. Your job is to make the important aspects of that model manifest visually, and usefully manipulatable.
Knowing anatomy, by the way, doesn't necessarily help you draw. I've had doctors that specialize in anatomy in class and they were terrible at drawing until they learned to see. They could recognize and label the parts, and they could even do a simple 2D schematic type diagram of muscles, but put a real 3D, foreshortened person in front of them and they were just as awkward as anyone else.