It may be that only a small subset of people will talk, but it's not necessarily the case that you know which subset beforehand. When the software can handle it, it's much easier to have everyone join a single call than it is to make sure that the right three people and two meeting rooms have access to talk, and guess which one other person out of about 250 might be called on to provide more context on an answer.

And I suspect that for most people -- including me -- Zoom accounts are "effectively unlimited". I wouldn't expect that many people to attend one of my meetings. The Internal Events team have licenses that allow for more attendees; I have a 500 attendee limit and I doubt I've ever gone above 50.