That's because of antiquated management structures. Look at Gore and Associates (makers of Goretex) for a modern corporate structure. Elements of a corporation should be almost completely independent and the only reason they are part of one entity is because of synergies and cooperation.
To give a toy example, McDonalds might grow and slaughter their own cows because they can better place them near their distribution facilities. Buying from a third party might create unnecessary overhead.
I'm sorta oversimplifying things, but I'd recommend reading
"Re-Creating the Corporation" by the late Russell Ackoff to appreciate why corporations exist (and why sometimes they shouldn't)
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