We just steamed a couple dozen tamales and I'm entirely baffled by the idea that they cook over hours. How? Why? What's happening? I initially figured I could check for doneness with a temperature probe. After less than 30 minutes, the probe read 212 degrees throughout -- and obviously the tamal cannot heat beyond this point as we're steaming. It was doughy and undercooked. We left the tamales for another couple hours, as instructed, and voilà, the husk fell off neatly and the tamal was solid. I don't understand. 212 degrees is well below the point where the Maillard reaction takes place. Is something else chemical going on? Is it purely physical -- maybe as simple as water boiling off? I'm at a total loss as to the science behind cooking tamales, and it's driving me crazy. 
There are dozens, if not hundreds, of things going on, physically and thermodynamically, during cooking. The Maillard reaction is not especially important. It is just one of several types of browning, and it only serves to improve the taste somewhat, not to cook your food through. The most important types of chemical and physical processes during cooking (and by "cooking", I mean applying heat to food, not other modes of preparation) are the denaturing of proteins, the breaking down of cellulose and hemicellulose in plant cell walls, the hydration and gelatinatization of starch, the dissolving of different molecules (e.g. sugars) in your cooking liquid or in the "own juice" exiting the cells, and the changes in aggregate state of lipids (which are much more complicated than the three phase transitions you learned in high school, with their amorphous solid state and tendency to emulsify). This is a rough list of what makes noticeable changes to texture and digestibility; there are also tons of reactions which influence flavor, or are unnoticeable by the eater. Also, you seem to think that chemical and physical reactions are quick. Maybe high school chemistry is also to blame for this; the teachers tend to demonstrate quick-and-spectacular ones; sitting around watching iron rust does not make for good teaching. In fact, many chemical reactions are slow, and some of those in cooking are also slow. In many cases in the kitchen, slow cooking is even preferred. It creates a much nicer, rounder taste, and sometimes a more pleasant texture. Tamales are intentionally designed for slow-cooking; both the steaming (which is slower than boiling) and the wrapping in leaves slows down the heat transfer, and ensures that they can sit around for a long time at heat without overcooking. So yes, it is perfectly normal for them to need a few hours even for the reactions which would be quicker in other dishes, such as the starch gelatinization, which is responsible for the sensory transition from "doughy dough" to "cooked-through-dough". 