Software was distributed in source format long before compiled binaries were the norm. There's a reason every Unix shipped with a C compiler you know.
This practice predates PCs by some years.
Open Source (tm) merely tries to codify enforce the tradition of sharing and cooperation via legal means.
Perhaps more apocryphal - Open Source (tm) seeks to prevent what Bill did for his first BASIC interpreter - which was take a public domain implementation, clean it up a bit, and sell it as his own work.
IOW, Open Source has had Bill's personal number for some time.