Because it’s way more precise than pouring liquids into a beaker (which can be off by 10%), much easier to judge than bending down to judge a level, and since with water 1 ml == 1g (yeah, metric system helps…) fairly easy to calculate. All other liquids commonly used are close enough.
Also what the fuck is 3 cups of flour/rice/salad (?!) even supposed to mean. Loosely packed? Compressed?
1ml of distilled water in perfect conditions = 1g ... but it's still close enough most of the time to not matter. Other liquids could be off by larger margins.
You probably won't be surprised to learn that I am very much a James Hoffman disciple, between that and being a foodie I was always destined to be a weight supremacist.