FYI, Spirit was originally part of Boeing. They're walking back a strategy of outsourcing that is said to be one of the causes of their recent QC problems.
The main theme for Boeing in the past few decades has been lowering costs. Be it by forcing the hands of unions to lower salaries and benefits, or moving production to non-union places (787s in South Carolina), or offloading as much design and manufacturing work to subcontractors as possible, spinning off "unrelated" business such as manufacturing. Most of those were absolute disasters.
Spirit is a public company [1].
Boeing spun out Spirit to be able to fire all the experienced union employees and move the work to a state where they could hire non-union people, all part of cost-cutting.