Think of how wasteful and inefficient multi-vendor rockets are as a concept. What complex machine would you engineer in such a way? Would you have the government, rather than buy cars from Ford, GM, Tesla, etc, instead contract out the production to one company for the motor, one for the frame, and one for the interior and instrumentation?
It was the only way to do it at the time, no company would have had the capacity for such a project, including reserves for damages. And even in private businesses it is common to outsource specific elements to external suppliers. The Saturn program was massive.
