Because often the sign includes a letter from the word. For example, “water” is signed by making a W with your three middle fingers and bouncing it against your chin. That wouldn’t work in other languages - think “aqua”.
But it doesn’t HAVE to be in a universal sign language!!
The symbol for ‘drink’ is a fist with thumb extended against the lips and tipped up as if emptying a glass or bottle. That isn’t the same as water, but close enough to get a beverage in any language!