So I don't know to what extent it would be possible to answer this question, but if anyone who's an expert on a limited area like Classical Greece or Medieval Europe would explain about their domain, that'd be great. I'd also like to hear speculations grounded in historical knowledge, if that's all that is possible.

So I understand a lot of ancient armies stressed combined arms, like Alexander's Macedonian army, medieval English armies with large numbers of archers, and so on. I also understand that a person who could not afford the top end of armor or a horse and come from certain sections of the population with unique skills might be more useful when relegated to an archer role rather than the role of, say, a Greek hoplite or armored knight. But why didn't Greek hoplites, armored knights, and other fighting men all carry some kinds of missile weapons along with their equipment for fighting hand to hand? Surely it wouldn't hurt to have them to use before the lines meet, or just in case you have a standoff situation with another set of fighting men who carried no missile weapons. If your weapon was encumbering, like a crossbow, you could always drop it before the proper fighting started, and I don't think there is any basis to claim that practicing archery (or throwing javelins, or slinging, or shooting crossbows) degraded your skills in hand to hand combat, or vice versa.