Because they're really, really bad at using the volunteers they do have in any effective way. They're really, really bad at getting the people who say "oh, I could help a little," and finding them little bits of help they can do. AFAICT, for many of the volunteer jobs at AO3, there are only two modes, with no medium: either overwork yourself to burnout; or do a minimal amount of stuff with no clear path on how to do more and feel like you're unneeded/unappreciated. I mean, the mentoring is great, if what you want is to be well-mentored into a burnout job! Otherwise... yeah.