You can filter the opportunities to only show "Event" types (as opposed to "Position" types) which are things like one-off volunteer events like this one: https://www.volunteer.gov/s/volunteer-opportunity/a093d00000...

But in general, I agree with you. From what I can tell, even though this is just broadly called "volunteer.gov", it really seems like it's specific to the Bureau of Land Management/National Park Service, and is missing a large amount of types of positions that I think most people would expect when they think "go volunteer".