Because he actually said it, according to this: https://en.wikiquote.org/wiki/John_von_Neumann#:~:text=Young....

Is it not?

> Is it not?

I'm not sure I understand the question. Yes, it is often attributed to him. If you mean "is it not true that he said it?", I don't know, but my suspicion, as with many such famous quotes, is that he probably at best said something like, or reminiscent of, it.