The inability to play by ear or, by definition, improvise, baffles me. I know everyone is different but to not be able to play unless someone has written it down is, to me, just weird.
The band in which I used to play had no music written down at all, not one jot (except the very first piece I learned because the band didn't know what I would need when I joined -- I didn't need it). In the end we had two and half hours of original music and could do three sets without repetition and every note played by ear. I'm not bragging, I just find it difficult to understand why everyone can't do this. Even my old music teacher couldn't play a note unless it was written down.