Geoffers,
I'm pretty sure that its because the majority of checks are not as objective and independent as they should be.
I once went to a CRMI symposium at the CAA, attended by company LTCs across the spctrum of the industry - I guess about 150-200 of them. The question was asked whether any of them had EVER failed anyone from their company on a CRM assessment. Surprise surprise - not a single one, ever!
I would imagine that company line checks and OPCs go pretty much the same way.