Presumably - otherwise they wouldn't be doing it - but it often doesn't take all that much to flip a volunteer from "Okay, this is cool, I can help other people and learn some stuff as well" to "Fuck this, it's way more trouble than it's worth." Top amongst this is when the people you're helping expect you to give them free work.

Jobs compete with other jobs, and most people expect that they'll have to do some unpleasant things in their job. Open-source & volunteer work competes with hobbies, and there are many hobbies where you never need to deal with demands, unexpected work, and interpersonal drama.