That is possibly the crux of the problem. When you have massive CRC turnarounds of 2-3 hours between flights then the disparity between duty hours and flying hours grows ever greater. Don't forget that the 900 hour limit is based upon flying hours only and not duty hours.
Exactly. It explains why some duty days are for example ten hours long with only four and a half block hours. It could become a lot more flexible and efficient. If you were to suggest an improvement with fixed links certain crew would never approve as they are "far too tiring".