Why do very religious men wear long black garments only, even in summertime?
Black, scientifically, is the absence of color. Wearing black only indicates lack of concern for color and other dictates of fashion, and thus helps keep priorities straight. In old sociological terms: to be inner-directed rather than other-directed. Anyway, it certainly eliminates the pressure of deciding what to wear each morning!
Long garments are a sign of respect. Nowadays, most people wear them only at the fanciest of affairs. Some religious Jews wear them only on special occasions, such as Shabbat and the Festivals. Others feel that every moment is a special occasion, because at every moment one has to be constantly prepared for prayer, Torah study, etc.
Now, if there are good reasons for wearing long black garments (at least, you'll grant, in the minds of those that wear them), why should summertime with its higher temperatures make a difference? If you were invited to a formal affair or to an important meeting that you would wear a suit-and-tie or a long dress in the winter, if it were in the summer would you wear a tank-top and shorts instead?
"Aha!" you say. "My summer outfit would be made of much lighter material." Believe me, the thinnest cloth you will ever encounter is that black stuff draped over some of those very religious men. And if you say, "Well, white would be cooler still," my answer would be, "Maybe it is not the most important thing in life to be as cool as possible" (pun intended).
By the way, in this latter part of the question, about comfort in summer, I detect some chauvinism. You look for overdressed-for-summer religious people and see only men! What about the long sleeved, stockinged, and bewigged women?
Seriously: those long black coats you see on the men is at most a custom. For women, these matters are tied up with the laws of Tzniut (modesty of dress and behavior). Varying traditions and interpretations play a role too. Thus, the different "dress codes" traditionally adopted by different communities. But covering the hair (for married women) and the body (for all women and men) is a matter of Torah law.
Some women wear a wig because they feel it is ultra modest, as it securely hides every strand of their own hair. Others prefer scarves and the like because they feel wigs are too natural-looking and attractive, which is the very reason that yet another set of women prefer wigs to scarves. I guess from every perspective, wigs are the hottest items (again, pun intended). On the other hand, some women wear scarves or hats in a manner that allows some hair to show, relying on the authorities that permit such and not wanting to appear too extreme.
I once overheard a conversation where a girl in shorts asked a woman in stockings on a 90 degree day, "Aren't you hot?"
The latter shot back, "Aren't you hot?"
"Okay, so I’m a little hotter."
She didn't add, "But I don't care because it’s worth it," but you could hear it anyway.
The person who suffers most from heat is not the one with the heaviest clothes -- it's the one with nothing else to think about other than one's own comfort. Next time you get caught in a heat wave in Israel or New York, look at faces as well as clothes, and see who seems to be bearing up the best.