This answers my question, and I'm now pretty convinced being a volunteer is not that impactful. Thank you!

my guess is suicide hotline volunteers aren't that stellar

I thought this because it's probably not very enjoyable and of course you're not paid. So I assumed there aren't going to be many people putting in tons of effort to be a great volunteer.

Why are people on the forum better?

I'm not sure if they are? I also don't think this matters to me, and don't think this is how career recommendations in EA work. 

Career recommendations I see primarily consider how much impact you have already assuming you're amazing at your job.  For example, being an excellent communicator/writer or a good entrepreneur are career recommendations I've seen quite a bit, and I'm not sure how great forum users are at these. If there aren't enough people like this on the forum, there will separately be some other post saying "We need more people who are good at this in EA."

I separately also think the skills to be a good volunteer are teachable and forum users are willing to put in lots of effort to get good at them. 

Ah, I did not realize the US ones are larger, this also makes sense.

I think my final skepticism is that US schools end much earlier than UK schools. I don't think they have finals in late May/early June (except maybe Stanford?). So I'm skeptical that there isn't a way to do this so that all final exams are avoided?

Ok, this makes a lot of sense! I wasn't sure if it was even a consideration. 

However, why not the US in May/June? My impression was that finals hold a lot less weight there than in the UK, and the US academic system weighs grades a lot less in general. People care a lot about exams in the UK.