"Diversity" isn't an abstract number we try to grow, it's a word to describe human beings who are extremely capable of doing a thing that they're not allowed to do because of the shade of their skin tone or what side of the tracks they grew up on. You want diversity because living in a world where people get to do what they want is a better world than any alternative.
You also want diversity because you want the best people for a given situation, and the odds it's actually a bunch of rich white kids that are the best at being doctors and lawyers and whatnot is basically zero.
But I agree with my sibling commenter; ChatGPT will give you the best form of this argument, better than anything I can write.