The larger the creature, generally speaking, the more "evolved" they are.
Just a nitpick, but what NW_Kohaku is referring to isn't a hierarchy of evolution like the thread seems to think, but rather that the larger a creature is the more complex the creature.
Remember that DF is a world with a creature that can apparently metabolize evil as its primary food source. There's no need to bicker about scientific accuracy unless creatures become modeled differently in the future (right now, they're just a series of tissue layers and organs, with no further details on their biology).
Back to the topic of the thread; chitin actually is edible for most mammals (you eat it in mushrooms). Most exoskeletons however are not pure chitin, and include elements like calcium to make the exoskeleton more rigid and durable.
The main reason humans don't mill chitin into flower and make Exoskeleton Bread is that there aren't a ton of good sources of Chitin. Most arthropods have highly nutricious protein anyway, so typically we just eat that and throw the shell away instead of spending the effort to collect and store the potentially hundreds of exoskeletons needed to get enough chitin to make it worth our while, develop a process for turning that into a more palatable food and refining out undesirable elements, and then talk people into eating it. It's not about impossibility, it's about effort. As an athropology student, I should point out that
only in marginally habitable environments is every possible calorie processed and consumed. We're an inherently lazy species; most hunter-gatherer groups have a capacity to create a surplus and don't because it's too much work. Just because we don't do it doesn't mean it's impossible.
I've seen plenty of mods which turn the current waste materials (feathers, chitin, scales) into crafting goods, but incorporating it into the base game is something I'd support. Each fort I've played has only used a small fraction of the available industries anyway, so more options really only makes the UI marginally more cluttered. It'd be worth it to give players options.