There is a vast number of sysctl in xnu that have not really been re-examined in over 15 years. Many tunings date back to the spinning rust drive era (for example). There are plenty of examples like this.
Disclaimer: I worked at Apple and poked xnu a bit.
The rationale Apple gives is something about using kernel resources, but a normal file descriptor also uses kernel resources, so I'm leaning towards implementation laziness or legacy like many of the examples here.