Most, if not all, of the lettuce I've seen has been uncooked, usually as a salad, wrap, or garnish. I don't recall ever seeing it being heated, even though visually similar leafy vegetables, like cabbage or kale, are often cooked. I have a feeling that this has something to do with the lettuce's water content, but that's just idle speculation.
Why isn't lettuce usually heated as part of the cooking process? I'm mainly looking for what happens when it is cooked that discourages people from cooking it, but I'd also like to know the scientific explanation. Instances of successful lettuce-cooking, if such exist, would be welcome, as well as explanations about what makes the cooking work in that particular situation.
Best Answer
First of all, visual similarity has little to no importance in cooking. The way an item is handled depends on its texture, taste, flavor and interaction with other ingredients, and it is rare that these are connected among similar looking substances. When it happens, it is because they are related chemically, physically or biologically, not because they look alike.
Now on to the lettuce, and why we don't need to cook it. There are a few reasons why we cook vegetables.
Because they are too hard to chew comfortably. That one doesn't apply to lettuce, it has very tender leaves. In fact, once you cook it, it goes not from hard to soft (as does a, say, carrot), but from tender to limp. On the other hand, it can be useful for cabbage. Try to wolf down a salad made from cabbage only, especially if you are a medieval peasant with no access to a mandoline (so your wife didn't cut it up in 2.5 mm stripes, but hacked as good as she could with whatever knife she had). It is possible, but requires much more jaw work than spooning coleslaw.
Because we want to mellow their taste. Joe's answer covers this, there isn't much reason to mellow the taste of lettuce.
Because we want to neutralize toxins or irritants. Plants like beans and potatoes come to mind in this category, but this doesn't apply to lettuce (neither to cabbage).
To kill off germs. Traditionally, this hasn't been a concern with most vegetables.
Because we like a filling, warm meal. This is by no means a universal preference, but the more you get into the North, the more likely it is that the main meal is served warm. Cabbage makes for an excellent main meal for said peasant - easy to grow in abundance, relatively carbohydrate rich, especially before starchy crops like potatoes and maize got widespread. It is also easily preserved for the winter (and after imperfect home fermentation, the taste mellowing part becomes important). So it is frequently cooked to soups or casseroles. Lettuce, on the other hand, is not really a main meal. It has much less calories per volume, and also per unit of cultivated land. So it is eaten in addition to the main, filling meal, not as a main part of it. So there is no need to make it warm too. Note that there are examples for cooked leaf soups without much calories, but these are poor man's food, and made from leaves usually foraged, not cultivated (nettle, sorrel, dock). They are used more as a diversion against hunger than actually stilling it, when there are no resources for better (= more caloric) food. They do deliver micronutrients though. I don't know why lettuce isn't commonly used this way, but I suspect that there is no wild lettuce in the places where nettle and sorrel are eaten. Of these, only spinach seems to have survived the cultural shift to centralized food production.
Because it is a structural part of some cooked dish. Other answers mention wrappers. Its shape lets it be used in this way, but it is a poor wrapper, both tastewise and in terms of durability, leaf size and thermal isolation. Wherever you live, you are likely to have a better wrapper lying around.
As the other answers mention, it does get cooked from time to time. But it is more of a whim, or a wish for diversity I guess. None of the usual reasons for cooking other vegetables applies to it. You can still cook it if you want to, but it is better suited for a fresh salad appilcation.