Back in the 80s it would have seemed bizarre to do otherwise. You used whatever OS the manufacturer supplied. It was part of the computer. That was as true with Microsoft OSes.
Apple choosing to make their own OSes instead of buy them from an outsider supplier was in those days no different from e.g. building displays or keyboards rather than buying them from suppliers.
Speaking of the 80ies, Apple is the one that wanted to control both the OS and the hardware (still true today, from phones to computers), and Apple is the one that sued MS over 'look & feel' issues.