Not to mention that a lot of majorly used open source stuff have even less developers (despite being nominally open to all to give a hand/debug/etc) than a typical macOS component.
To the point that the GTK+ maintainer lamented that it was just him, a single person, doing most of the work, a few years ago -- and begging for others to come help.
Ubuntu w Unity 7 and Ubuntu MATE are great, but standard GNOME3 is garbage - too many stupid UI warts.
The cause of many of these bugs is an aggressive annual release cycle and fixed release deadlines, not necessarily insufficient QA. Making a significant part of the QA function voluntary wouldn't help with meeting tight deadlines or dependability of the QA process.
The general point of expanding QA resources might help, but it's certainly not a given that making that on a voluntary, best effort, when you get round to it basis would help with the timeliness and reliability of releases without sacrificing some of the other goals Apple and their customers holds dear.