Well, the simple answer is that its predecessors weren't "more basic". Latin has actually lost several noun cases and regularized its verbs significantly from the way Proto-Indo-European worked.
In general, though, naturally-evolved languages are never more "basic" or "sophisticated" than others. It's somewhat of an axiom in linguistics that all languages are equivalent in their expressiveness, and while there's been a lot of debate over what exactly that means, recent work with information theory supports the idea that all languages convey the same amount of information over time. More information here (disclaimer: I wrote this article); the short version is, while English might convey more information per syllable than Japanese, it can't convey as many syllables per second, because losing one of those information-dense syllables to background noise would be much more of a problem. Classical Latin, in particular, appears to convey about as much information per syllable as English does.