The problem with inventing languages is that we don't fully understand natural languages. For example, there is no satisfactory formal semantics for natural languages. Categorical grammar and Montague grammar are some inroads into formal semantics, but they don't get very far. The work of Roger Schank got further in terms of expressiveness, but then fizzled out, probably because it was ad hoc and lacked an underlying mathematical structure. So inventing a language is working in the dark.

There are some natural languages which were in part designed in a top down manner, like Mandarin and Indonesian. But still, on the scale of natural to artificial, these are much closer to natural languages. They were both based on some other natural language(s), borrowing words and grammar from them.