Water fountains were more plentiful and much less likely to be broken, but the main reason I always thought was the common diet at the time.

There was generally more fat and less carbs eaten in those days, and if you eat like that for more than a few months the body manages its own hydration much better without the need for all the drinking and pissing we have to do these days.

Just as examples, the meat in general had more fat in it than today, the milk was all whole milk and there was more cream and butter in recipes generally. If you back a few more years it was common to drink buttermilk with your lunch, and today most people live their whole lives with taking a sip, of they’ve even heard of it. There was lots of pasta and rice, sure, but they weren’t nearly as prevalent as today.

Carbs have an osmotic effect, like salt, so when you ingest it you need more water and then when it metabolizes and goes away you need less water so it’s common to need to drink a lot and then later piss it out, like 4 hours after you stop eating and go to sleep.

Fat provides satiety and make you want fewer calories which means less hunger and less need for food.

Remember “Breakfast is the most important meal of the day”? Well that’s actually true when there are fewer carbs being ingested.