There are very few people who do stuff for the sake of doing stuff. If you are a volunteer you’d know because basically there is always a shortage of them, never once a volunteer has been turned down at least I never saw it
People don't volunteer because they want a boss tell them how to spend their free time. They volunteer because they want to do something meaningful in the way they prefer.
Volunteers want autonomy. If you can't provide it, there is no room for new people in your organization. To attract more volunteers, you often have to restructure the organization and change the ways things are done. And sometimes you have to get rid of existing volunteers, because they have too dominating personalities, are too toxic, or are too attached to the old ways.
Turning down volunteers is quite common in successful organizations.