My DD is about 4 months older than yours and her why why why why why why why stage started just about a week ago. She does it on purpose to irritate me.

Prior to that, she asked why all time time but she used it sarcastically then I realized she was mocking me since I tend to do that.

I don't know if that why stage is a language developmental milestone. It could simply that it's not in her personality to be asking why all the time. If that is the case, then lucky you.