why is there this Reaver asteroid belt where they all hang out-- way too closely to one another, but anyway-- waiting to entrap some luckless, what, space tourists?
Because this is home, and they've been moving out from there, but some are faster at it than others. Also, as others have mentioned, they may sometimes bring their toys home to play. On the other side, we've been shown time and again that there are a lot of rubes out there. Even on Serenity, not everyone knew that it was a dangerous area until Wash told them. Some group heading off to settle a moon might not realize that it was an area to be avoided.
That brings up something else, though. I didn't notice actual screaming. What I did hear sounded like what River heard in Bushwacked, sort of unrealistic sounding screams, so I kind of assumed that anything we heard was leftover psychic screaming for dramatic affect.
We're given the impression that they're relentless killing machines. As in, they don't rest, they don't lurk, they don't scurry, they don't have a homebase. (IIRC from the show, they were actually encroaching on civilization.)
Not exactly. Wasn't it the pilot where Mal says that maybe they're full? I don't think that the reavers thought that they were dead, just that they (Serenity) weren't running and they (reavers) weren't hungry.
Except the logic that most of those victims screaming on their radios probably should have known to space themselves rather than be captured alive by the Reavers.
Above, I wrote how I didn't think that there were necessarily folks screaming then and there. However, even if there were, we've been shown more than once that most people didn't really believe in reavers, that they thought they were made up stories. It stands to reason that if you didn't believe they were that bad, you might try to live long enough to discover that you were wrong, at which point it might be too late to do anything about it.
eta: I type too slowly.