While, yes, (으)ㄹ 거예요 is used to indicate that something will happen in future, suffix -겠 has various meanings. For example, it can show intention, guess or assumption — all of these, implying that you’re subtle and careful with your words. So, I’d say 모르겠다 has a meaning of “I don’t really know/ I am not sure” Whereas 모를 거예요 can roughly be translated to “I won’t know”. Meaning the two grammars are different