For NYU and Columbia Why X's, what I hear is that they kinda already know why people want them so it might be redundant to add an additional document on this, but if you have a really compelling reason to write one, don't hold back.
As far as Columbia goes, I believe they specifically asked for you to explain “why Columbia” in the personal statement. Not sure if that’s helpful or not, but that’s what I did for my application to them.
Anedoctal of course, but went complete for Columbia early Feb/late Jan and got in recently. This is a slow cycle so late apps really aren't as punishing as they have been for previous cycles.
Did you write the optional essays? December isn't early, but it's not super late. Getting WT from Michigan and Duke with those numbers makes me think the schools didn't think you were serious about attending.
There are optional essays that you should probably ... example really cares about the optional essays. The NYU scholarships I ... mentioned also have optional essays. So did Stanford. Yale ...