It's more or less impossible to get your AI to drift as they are made to race. They will try to slide as little as possible even with the most slidy setup... or spin out of control.
Try making the rear brakes stronger than the front, if you do it right, bots will tailbrake into the corner, which is not exactly drift, but it's the best you can do
Well on a few versions back still being on demo with the XRT. I saw someone that strangly enough, trained his AI that good, that it was drifting like a pro.