Americans don't eat vegetables though

They also eat plenty of salty processed foods.

Which helps with water retention, so...

Agreed. Being in the Midwest US, my intake also varies widely, depending on weather or season, physical activity, and the foods I've been eating.

I'm not entirely dismissive of doctors, be they European or American, as most I've encountered do have the patient's best interest at heart. But they are also human, and it is very easy to stick with the safe and easy answer rather than do the work to find the real answer. So when I hear claims like that, I immediately doubt them, assuming it is placeholder information because we do not know the actual answer. Unfortunately, a lot of our media in the US considers such "placeholder information" to be actionable, and ends up convincing the public (including doctors) of its veracity.

European recommendations:

https://www.eufic.org/en/healthy-living/article/how-much-wat...

I always laugh about those ridiculously large water bottles American carry and how they remind you all the time that you must drink water as if I did need it. I wonder why that happens.