We aren't even conscious about most of what goes on in the brain; consciousness is still a poorly understood phenomenon, but we know that lots of brain activity happens without conscious awareness. Most of the regulatory functions of the body are included in that unconscious activity, but so are some processes closer to what we do in day to day life.
For example, if you are experienced driving a car, you are able to do most of the general functions (staying in a lane, for example, or starting to brake at an intersection) without much conscious input, allowing you to have a conversation simultaneously. You only need to be strongly aware of what's happening in an unexpected situation.
Similarly, muscle movements come from executed programs that are learned through experience (especially for humans; some of these movements are innate). When you raise your arm above your head, for example, there are muscles in your arm that move, but also muscles in your back and legs have to adjust to prevent you from tipping over with the new weight distribution, you might lean your head out of the way, all of this is done without you thinking about each one of those individual muscle movements: you are just conscious of the whole general movement.
Regulatory control of things like your heart rate or digestive system is even less out of conscious control.
Brain-computer analogies are often fraught with peril, but in this circumstance I think it is reasonable to use one. Your computer is able to perform all sorts of calculations, it even has software that allows you to communicate with a network of other computers to bring you StackExchange. But even if you download computer science books onto your computer, it has no idea how to assemble a new computer from scratch. It can display the text that could allow someone else to do it, but it is completely unnecessary for its day to day functioning to know how to replicate it.
So back to your original question: why don't we consciously know how our bodies work? Because it's completely unnecessary to know this for normal day to day operation in the context in which animals evolved.