There's a lot to respond to here and I'm posting from my phone, so please bear with me since formatting and proofreading are a nightmare.
So all the soldiers fighting in real world wars today are dropping like flies? The crusaders* got out of the middle east because they couldn't stand the heat?
I never said people would be dropping like flies and putting words in my mouth or using hyperbole to make your point do you no favors. I simply stated a fact of living in the real world: if you over-exert yourself in extreme heat the consequences of doing so are not pleasant. And by the by, in controlled circumstances, i.e. not combat, the military does take the possibility of heat casualties very seriously and will try to adhere to the guidelines whenever possible.
I get it. Jungle hot. I've backpacked in the jungle (Peru) and it sucks. Especially when you go in June (which is supposed to be their winter) but it's unusually hot and humid. But you slowly acclimate, take it easy and, no we didn't have people dropping like flies despite carrying backpacks.
So what you're saying here is that you acclimated, hydrated properly, and "took it easy" when it came to exerting yourself? Sounds an awful lot like following guidelines for being active in extreme heat, the kind of thing that won't see you end up a heat casualty.
And we all have anecdotes. Half my players were in the military at one point or another. One of them, just before he got out a few years back, had his unit sent to Louisiana for training exercises. His commander decided to ignore the guidelines for extreme heat because he apparently thought he was bad ass... so many soldiers in the unit got sidelined, approximately 40% of an entire battalion, as heat casualties the entire exercise had to be called off. See? We all have anecdotes.
In a world where arrows bounce off the barbarian's pecs because he's "tough", why is it so important to penalize people in armor based on suppositions and guesswork of how it should work?
Because some people like it. For some people trying to emphasize something like the oppressive heat of being active in a jungle is something they want to do with more than just words, some people want mechanics that will back that up. There's nothing wrong with people wanting to emphasize points of "realism" that interest them in their games even when arrows are bouncing off of the barbarians bare chest as a dragon flies overhead. We all play the game in our own way. My question to you, is why do you care?
You seem awfully concerned about what other people want to do in their games. No one here is claiming that penalizing characters in heavy armor is the "only" way to play, or the "right" way. They just seem to be stating an interest. No one is forcing you to use this in your game and I'm pretty certain no one plans on kidnapping you and forcing you to play at their table by their rules...
As a final aside, because I'm not sure if I've actually said this yet in the thread, while I'm interested to see if the AP says anything on the subject I very much doubt I'll even penalize the heavy armor wearers regardless. I run what would very much be classified as a beer & pretzels game, we generally don't sweat this kind of thing unless I feel like it adds something interesting to the game. Just making that point because odd as it might be, despite everything I've said that seems to indicate heat + heavy armor is bad... I don't actually have a horse in this race, I'm just adding in some interesting discussion ppints... at least I think I am.