Control of knock on production engines can be confused with internal
high speed mechanical noise of the moving parts as the engine wears.

In effect, if it ran closed loop continuously, as the engine wears over time
(even after a few 100 miles) the high frequency component of the
noise confuses most knock systems that knock is active when it isnt.
The result is ignition retard and power loss.
Nearly ALL ecu's that employ knock control disable the feature after a certain rpm
and over a pre-determined boost pressure level.

Another reason for this is....
If the knock sensor failed during high load situations, the closed loop action
could in effect have the reverse effect of advancing the ignition as it may
think there is NO knock and this COULD result in producing DET.