Linguistic drift IMHO. Formally, "dice" is the plural of "die". Less formally, "dice" are cubes with numbers on them and "die" is a completely separate word.
I never asked, but I want to believe their internal representation of “1D6” is one-dice-six.
Sorta like reading “1 x 6” as “one _times_ six” instead of “one _time_ six”
Words are fun.