Except we're not monolithic. We each have our reasons, and one person's rationale for open sourcing (and with what license) may vary by project and with time.

> It may be the case that in 2024, lots of people "do open source" as part of a business strategy. But I can assure that it was not so in 1986 [...]

Not explicitly, perhaps, but already in the 70s there was something of an understanding of mind-share. Building mind-share is easily (IMO) the most important reason to go open source for any given project, though there's others too.