I would suspect that many companies use vendors like McMaster Carr, Grainger, and MSC from convenience. At a company that I worked for some fifteen years ago, we would write up a request for purchase and send it down to Purchasing. The go-to supplier for machine tooling was MSC, because the purchasing agent was familiar with their product offerings, the product line was fairly complete, and the sales rep was really nice when he stopped by. The same was true with raw materials and McMaster Carr.
ISO 9000 throws another monkey wrench into the mix. Most Quality Systems require using approved vendors. The last company that I worked for, a global Fortune 500 company) discouraged setting up a new vendor, stating it cost the company $500 to set up a new vendor. If we wanted to order something from an unapproved vendor, we either had to put the order through an approved third party who would order for us for a healthy surcharge or put the order through on a company credit card and take the heat when we had to justify the credit card bill. (Fortunately, my supervisor was understanding and shouldered this burden for us).
Companies like those mentioned rely on this strategy to ensure customer loyalty and numbness to high prices.
