It might be related to the fact that Toyota championed "Just-in-time manufacturing" which requires tight control over the supply chain. You have to be able to trust your suppliers to deliver what you need on time and in high quality. So you must cultivate close relations with your suppliers over many years. They almost become your partners. You can't just stop working with one supplier and switch to another on a whim.

"Toyota believes in developing mutually beneficial, long-term relationships based on mutual trust with all suppliers. To foster that trust, we pursue close and wide-ranging communication to share our business knowledge to enhance our business relationship." (https://www.toyotauk.com/toyota-in-the-uk/supplier-relations...)