It is because you don't hear about the cases which don't slip up.
The Zodiac Killer; never been caught. "D.B. Cooper", never caught. The presumed murder of Jimmy Hoffa, unsolved.
Sure, but there is really no shortage of known unsolved crimes.
> The police does not put out press releases repeatedly with 'we got no leads'.
They do, in all the cases I listed they've asked the public for help for years. Certainly they don't do that for crimes they don't know occurred, but there is no shortage of known crimes that are very publicly unsolved.
For that matter, there are also a whole lot of missing person bulletins soliciting information from the public. Many of them might be murder victims, but it isn't known whether they are really alive or dead, let alone murdered. The public is nonetheless asked for information.
That's certainly the nice way to put it -
"If you're murdered in America, there's a 1 in 3 chance that the police won't identify your killer. To use the FBI's terminology, the national "clearance rate" for homicide today is 64.1 percent. Fifty years ago, it was more than 90 percent." ... "Criminologists estimate that at least 200,000 murders have gone unsolved since the 1960s"
https://www.npr.org/2015/03/30/395069137/open-cases-why-one-...
When the existence of such a serial killer is recognized, it tends to make the news at least regionally. Sometimes they become internationally famous for many years. But to my point, the public hearing about it is not contingent on the culprit being caught. If anything, the ones who are caught fast and easy tend to make the least amount of news. You can 'juice' unsolved crimes for stories a century after the fact, but stories that follow the "husband did it and we caught him" format tend to disappear from the news after the culprit has been sentenced.
Might just make for a blockbuster movie.
fbi says there are over 2,000 serial killers free in the US
we dont know nearly enough of them to know motivations or even assume thats a prerequisite
Source: https://ucr.fbi.gov/crime-in-the-u.s/2017/crime-in-the-u.s.-...
https://archive.seattletimes.com/archive/?date=19980804&slug...
That doesn't always work though - the crime may be something the public is aware of, and then the pressure to "close" the lead is even greater. There are numerous "big" crimes that the public has no awareness of though.
If the case is high profile enough, they will _always_ find someone to find guilty.
It seems statistically unavoidable that there are always going to be some big cases that go unsolved. It'll probably become less and less common - especially since it seems inevitable that nearly all DNA will eventually be traceable via genetic genealogy databases - but some crimes just won't result in any DNA or other significant evidence being left.
Or to make the crime legal by paying politicians
Unsolved Mysteries type shows and news stories would be all over that sort of thing.