It’s probably due to wanting the comforting thought that such murderous perverts are rare and weird “loners” that many people embrace the fiction that serial killers work alone.
In fact, in all documented cases, going back through the decades, many serial rapist-killers have had helpers.
These helpers range from amoral monsters in it for the money (like the teenagers who helped Korll) to coparticipants who shared sexual fantasis of a deranged murderous kind (the Wests, Bundy’s cult, John Wayne Gacy’s “others” etc.) to people involved in the snuff film satan/drugs matrix (Manson, Son of Sam etc.)
The lapdog media is its usual hype laden unreliable self about this topic – turning serial perverts into supervillains. The thing is, even accepting the lies of the media and the law enforcement bozos who take decades to catch these monsters, when they try at all, supervillains frequently team up or have henchmen. Even in the lie, a little truth leaks out.