[johnhenry]

One important distinction between a clock an the mechanism is that clocks can be would up and run "by themselves". Perhaps this was the missing key? (Though, this would not preclude them from creating "mills" run by wind/water to display time.

Another thing to consider is that the concept of doing things at a specific time may not have matter to much of the worlds population at the time and, even though they were capable; there was no demand.