It's amazing that we're filled with nerves that can't directly provide us experience. When your stomach growls, it's because your autonomic nervous system is sending waves of contraction down your small intestine to clean out any food material that remains. But even though it's your own intestine, you have no power to start or stop this activity, as you may have discovered to your chagrin in a conference room at one point; you "find out" about your own organs' activity only indirectly, by hearing them as if they're coming from someone else, or feeling it incidentally in consciousness-impinging somatosensors on adjacent muscle or skin. This becomes even stranger when you ponder that we have more autonomic motor neurons than we have consciousness-impinging somatasensors. And yet, somehow, this mass of neurons certainly doesn't seem to be conscious. Why not?
I haven't seen any philosophical explorations of the nature of consciousness in terms of the ANS, but it may be profitable to ask what the autonomic nervous system lacks that the central nervous system has. Clearly a large network of neurons is necessary but not sufficient.
The question is not just why we can't consciously form migrating motor complexes in our intestines. They're connected to the wrong part of the brain for that, and there is no instance of smooth muscle which is under voluntary control. (If you can voluntarily move a muscle, it's striated; most of the rest are smooth, with the notable halfway exception of your heart.) As a speculative aside, smooth muscle is structured in a seemingly jumbled way relative to the machine-like geometry of striated muscle, but I wonder if this says more about the way we perceive patterns spatially and temporally than it does about the respective complexity of the tissues. That is, our space and time perception has developed for obvious reasons on the same scale as the voluntary movement of our bodies - of our voluntary nervous system. But the world sometimes becomes alien and even incomprehensible when we look at satellite images or super slow-mo videos; maybe in a real and objective sense there is a logic to the organization of smooth muscle but our pattern recognition filter is not designed to see it. After all, it took until the 17th century for someone to think of integrating information into a graph; there are many accurate ways to represent data coming in from the world, even if our consciousness doesn't automatically tie stimuli together that way.
Birds are another interesting problem. Where the ANS is missing a function (consciousness), birds are missing a structure, but still have the function. Even if they aren't conscious, birds are certainly intelligent, yet they have only a very thin cerebral cortex. Learning in birds occurs in the Wulst, within the corpus striatum. The cognate structure in mammals are the basal ganglia, which are islands of gray matter near the bottom side of the brain, surrounded by white matter. In mammals, they do have roles in learning and decision-making (especially the caudate nucleus and controversially the subthalamic nucleus, respectively) but they are certainly not sufficient. Why is the corpus striatum in birds adequate for learning? What design features does it share or how does it differ from the cognate structures as well as the cortex in mammals?