Precisely this.
It's almost a certainty that vast countries like China, India, Brazil and so on, with limited development and resources and huge, isolated, low-density populated areas are harbouring significant numbers of prolific, but entirely unrecognised serial killers.
Then there are the partially lawless States in Central and South America where violent crime and violent death are so commonplace that it will be entirely viable to be a prolific serial killer and not even make any attempt to hide your activity, but because the consequences are so indistinguishable from every-day events, law-enforcement will not recognise the results as serial-killings, or if they do, will probably attribute them to some other motive, gang-related, drug trade etc, than typical killing for the sake of killing. See Ciudad Juárez.
We have comparatively few serial-killers in Western countries now entirely because advances in policing and detection mean it's astonishingly rare to get beyond your 2nd or 3rd victim without being apprehended. It's much more difficult to actually meet the criterion to become a serial killer. Exceptions still occur, obviously, but it's changed days from the era of hand-written card databases and regional police forces who did not communicate.