Well, Apple could have chosen to develop something new from scratch instead. Most software companies at the time (10 years ago) would have not even considered building on top of an open-source project.
That said you should also not give too much credit to Apple. I think that Darwin was build on top of FOSS is merely because it was developed by NEXT, which was a startup and needed to save cost. The original MacOS was closed source after all.
What is up with people giving Apple credit for everything in this discussion? Darwin is composed of the Mach 3 microkernel, parts of BSD and NeXTSTEP. In other words, the vast majority of the work that has gone into it is not Apple's. It's merely a BSD-style distribution.
That said you should also not give too much credit to Apple. I think that Darwin was build on top of FOSS is merely because it was developed by NEXT, which was a startup and needed to save cost. The original MacOS was closed source after all.