grow up is a thing that will happen in the future. Why not use the future tense here? I am a little confused.
The word when is really the deal breaker here. It does in fact indicate a specific event taking the future.
I'll believe it when I see it.
You might want to argue that instances exist in which the same word, "when," does NOT indicate the future. Such as:
When I see you my entire universe fills with dazzling light.
Well, not quite. In this instance, the word "when" is short for "whenever," which is not the same thing.
Because English doesn't use the so-called future (the "will" form) in "if" or "when" clauses. *
I'm sorry, but that is the whole of the answer. One can speculate about why that has come about, but it's simply an arbitrary fact of English grammar.
*There is one apparent group of exceptions: with "if" (not "when") and second or third person subjects (not first person), phrases like "If he will wait for me, ... " do occur. But here, "will" is not the modal which forms the so-called future, but a vestige of the original meaning of "will" = "want" or "be willing". "If he will wait for me" means something like "If he is willing to wait for me". This form is often a sort of implied request.