Because a gallon is divisible in human unit. A quart is a quarter or a gallon, a pint is half that. 250ml is both exotic sounding and guaranteed not as exact when measured in halves.
And why would a quarter of a gallon be more convenient or 'divisible in human unit' than a quarter of a liter...?
Say, I walk into a bar. I order a beer. Here (in The Netherlands), since I didn't specify, I get a small beer of whatever's on tap. A small beer is 250ml-350ml, depending on the establishment. I could also order a half liter (500ml) or ask for a pitcher. So, just as in the US you don't order "can I have 128 fluid ounces of beer" you don't order "250ml of beer" in the rest of the world. Also, the fact that it 'sounds exotic' says more about you than about the system... and 'guaranteed not as exact'? Really? Let's say you have to measure a gallon of water, and I have to measure a liter. I just whip out a digital kitchen scale and measure 1000 grams. Which is EXACTLY the same amount as 1000 milliliters. Cause, you know, metric units are interchangeable. I double dare you to come up with a more exact measurement in Imperial units using simple kitchen tools.
> In The Netherlands [...] a small beer of whatever's on tap [...] is 250ml-350ml
Traveler beware: Just because it's true that a Dutch "vaasje" glass is 230-330ml[1] the actual volume of beer you get is at best 80% of that. The Dutch pouring practice of handing patrons a glass of beer with up to 1/4 receding foam on the top would cause a riot across the channel.
> [...]and measure 1000 grams. Which is EXACTLY the same amount as 1000 milliliters[...]
No, not exactly. Not even under standard conditions. It's one of the unfortunate warts of the modern metric system that the actual value drifted from exactly 1000, almost, but not exactly[2].
So, I'm only going to try to explain my view once more to avoid a million downvotes replying to all, but I'm not intentionally being difficult.
Most places, I think, most people don't talk in precise units at all. Give me a glass of water. Put in a pinch of salt. One scoop of coffee. Handful of peanuts. It is my personal experience that this tracks pretty well across any culture. In my opinion, having these oddball measuring units is an easier mental model. A couple tablespoons is human sounding. 30ml is not, even if you have a 15ml spoon. A cup of water sounds human, a quarterliter does not. I'm not trying to insult you or your system, I'm just expressing my own opinion, which as pointed out, probably comes from my own education and experience.
