I think you’ve been watching too much CSI or something. There is no way for police to identify something as a stranger murder until they’ve already investigated and determined the perpetrator of the murder. Suspicious disappearances aren’t even treated as crimes by the police in many cases, unless there is evidence of foul play, much less checked against some nationwide database. If there is evidence of a serial killer at work, the police can be VERY selective about releasing any information to the media, though the sensationalism of the media when they do get information might lead you to believe otherwise.
Serial killers often get away with it for decades, because the very nature of their crimes make them very hard to catch. It’s fair to say that police are better at realizing when there is a serial killer at work now than in times past, but they are still usually caught when they make mistakes, or by lucky breaks, than by modern methods of police work.
When I say murder, I specifically mean the murder of women with evidence of sexual assault.
Police can usually clear the prime suspects - boyfriend, husband - within hours.
When I say suspicious disappearances, I mean women who suddenly stop showing up for work or school, stop calling their mothers, stop using their credit cards.
Obviously, women who are drug addicts, or mentally ill, or prostitutes, or chronic teenage runaways will require more investigation.
My point is that being a successful serial killer is massively harder today than it was even 10 years ago.
And the crime data confirm it.