Sure,
Many companies model their systems to meet the requirements of the various "Quality" system requirements or specifications, (ISO/QS/TS...) and probably show evidence of compliance through their internal audit mechanism and management review.
While they comply with the requirements they choose for whatever reason (usually $$$$$) not to get registered by a third party source. These are usually your smaller tier II and lower subcontractors to tier I suppliers, at least in automotive.
Most companies are committed to a quality system that is effective, but it is a big step to quantify the "cost" effectiveness of formal registration.
ASD...