Because the car still gets warm. Moving or stopped.
Checked it over and everything is fine, and how it should be in the engine, so can only assume the intercooler is blocking the air.
The fan works to keep the car cool brilliantly, unless I'm stuck in traffic for a long time, then I have another fan on a manual switch.

Sorry, just saw the post above, I tried the cossie fan switch, it didn't help, I also have the puma thermostat, and alloy radiator.
Apart from draining the battery at the moment, having the fan on constantly doesn't seem to produce any other problems.
I also have a 4 litre supercharged Jag, on which the fan stays on constantly, it came out of the factory that way.