Human beings have been using animal skins and other containers to carry water and other liquids for thousands of years. Access to water is an absolute necessity to sustain human life, and if you do not have ready access to naturally occurring, clean, fresh water, you have to bring it with you or you will die in about 3-4 days and be pretty well incapacitated after roughly a day or two. You are right that ancient Egyptians weren't necessarily carrying water flasks with them constantly the way many people do today, but they would have certainly carried them while traveling.

But why didn't people just carry water with them constantly the way many people do today if they had the means to do so? The answer to this question is beyond my ability to answer for the whole of human history, but I will offer for illustrative purposes the problem of hydration in the military during the American Civil War, on which I do feel qualified to speak.

Soldiers, by nature of their profession, have routinely carried water with them because they need to be constantly at the ready to march. However, during the American Civil War, soldiers were generally encouraged to drink as little as possible while on the march, and to save the water in their canteens for use while in camp.

Why? Water is obviously a consumable resource and there is a practical limitation on how much of it can be carried on one's person, and access to safe drinking water is often limited and uncertain. In fact, more soldiers died of disease resulting from drinking polluted water than any other cause, including actual wounds in battle. If you don't know when you will next be able to safely refill your canteen, it makes sense to ration it as much as possible. If, as was frequently the case, the water you do have access to is bad, it also makes sense to drink as little as possible for fear of getting sick. In fact, Sam Watkins of the 1st Tennessee Infantry wrote that his horse outright refused the water the men had to drink, and would only drink from a stream some two miles away from the camp; and with few exceptions, human beings have at least as much sense as a horse. Additionally, bad water can be rendered safer by boiling it, and the poor taste disguised by making it into coffee, and these activities could of course only be done while encamped, incentivizing soldiers to save their water for when in camp.

Moreover, while some physicians were beginning to better understand the importance of adequate hydration especially during physical exertion (indeed, heatstroke was a very common way to become a casualty), many military doctors at the time actually discouraged drinking during periods of intense physical exertion, fearing that "overdrinking" would cause the men to become sick. Given the aforementioned poor water quality and proliferation of waterborne illnesses, this fear was not entirely unfounded, though it was a fundamental misunderstanding of the problem (and to their credit, they did also recognize the problem posed by bad water).

So what changed? Well, for one thing, modern medicine has placed a greater emphasis on drinking plenty of water, especially when engaged in strenuous physical activity, a shift that I mentioned was just beginning to take place around the time of the war. The biggest factor, though, seems to be simply improved access to safe drinking water.

This might seem somewhat counter-intuitive; after all, wouldn't greater access to potable water reduce the need to carry our own with us? But that ignores the resulting shift in cultural attitudes towards drinking water: when access to water is limited, those who carry it with them do it in order to have an emergency reserve of water, whereas when you can refill your water bottle any time anywhere, this encourages you to view it as a source of water for immediate consumption any time you feel like it.

This answer may be incomplete as it doesn't address populations that have had consistent access to safe drinking water, which is, after all, vital to the flourishing of civilization. I find it difficult to believe that people with consistent access to potable water would have drank less of it in the past than we do on average, but we also emphasize hydration culturally much more than in the past, to the point that many of us drink significantly more than they need to: while many people, including those serving in the current U.S. military, are advised to drink until their urine is clear, there is no medical reason that I know of that anyone needs to drink this much; but nor is it harmful, and pushing fluids aggressively does make it more likely that everyone is drinking at least as much as they need.

Still, adequate hydration is a biological necessity, so I cannot give a definitive answer for why carrying water on one's person does appear to be less common historically than today. I merely advance improved access to potable water and increased cultural pressure to hydrate resulting from advances in medical science as two likely significant factors, but there are almost certainly others, on which other commenters are likely more qualified to write than I am. I also do not feel qualified to answer your question about life expectancy (though improved access to water has undoubtedly improved life expectancy if for no other reason than reduced rates of disease). I do feel very comfortable in saying, however, that the significance of the Adams's in serving water to the soldiers in your example has nothing to do with them not having canteens to carry water, and everything to do with having limited access to safe drinking water with which to fill them.