..If pilots are so rubbish on the sim checks why aren't they failing and being sent for more training.
Simple. Pilots arrive for their sim checks on a pre-paid course to a set syllabus that includes a defined amount of ground school and simulator time. Usually there is no more money for any more training over and above that. And the commercial reality of the business is, if you start failing pilots on their sim checks the operator (i.e. customer) would just send all their pilots to a different training provider, one with less rigorous standards and more forgiving check airmen. The bottom line is, for the $25K recurrent course cost, they just need the completion certificate and a signature in the log book. The training outcomes are often secondary.
As an instructor and a very small cog in a much bigger multi-cog machine, my approach to the matter is teaching tail rotor and other malfunctions that might seriously bite you, well, there is not enough time for that. The training time is far better spent on teaching the recurrent pilot trainee what buttons to push and when, on a properly serviceable helicopter with no malfunctions, because getting that wrong is far more likely to happen and ruin their day than the tail rotor falling off.