Drive cooling fans always on

If the fans are controlled by a thermostat and not otherwise switched, I would submit that the drive is still hot even after switching 'off'.

It is usually an economic decision on the part of the VFD manufacturer. Fans come in two styles: continuous duty and intermittent duty. The bearings on continuous duty fans are different and if you turn them on and off a lot, the bearings get dust in them, severely shortening their life. So they actually last longer if they are run continuously. Intermittent duty fans have sealed bearings and don't have that problem, but are more expensive and use more energy. So add the higher fan cost and the larger power supply to run them to the added cost of a thermostat and you increase the cost of the VFD while decreasing the throughout efficiency, a specification that a lot of customers pay attention to. The only advantage to using the thermostat is to reduce the noise.