it's a matter of habit. 3l of liquids per day is what various doctors in different (european) countries have recommended me over the years.

Are we sure this is not just harmless and arbitrary information being parroted? Do we have verifiable sources other that anecdote? I find it hard to believe that there is just a single value for water intake across the massive biological spectrum that is humanity and expect to see a range when this conversation comes up. You're also getting water from foods, which I am sure is not being accounted for. Reminds me of the 10k steps a day that just happened to be "correct enough" to be believed and acted on. The truth is much more nuanced and depends on a number of factors in a person's physical health.

Without concrete verifiable findings, the best we can do is learn to pay attention to our bodies and drink maybe a little bit more water than we think we need to.