if NVQs are the true measure of competence why are they still not accepted by the majority of "construction" employers?

mickn2k asked the following:

Plant Operators within the CPCS have to complete an NVQ as part of the route to full competence. However, UKCG contractors will not accept the NVQ in isolation, but demanding a "CPCS card" for site entry and equipment tasking.
NVQs are accepted as a measure of competence within extractives and construction industries. Not working in the construction industry it would not be appropriate, or possible, to comment on the specific requirements of that sector. My advice would be to contact UKCG or CPCS direct in this instance.