I'm not a programmer but I'm pretty sure the spawn rate doesn't change...If anything I think that after the trick or treaters interact with so many people they'll go away...but I also think they're on a timer...I'm honestly not sure but if they're programmed to go inside after interacting with so many people then it'll just seem like the spawn rate is lower due to there being less people on to interact with them. I don't think anything depends on how many players are online. But I could be wrong. Like I said, not a programmer, wouldn't know for sure.

Tbh you guys specifically could just be really unlucky in finding trick or treaters.