Where do you live?
In the U.S. police will show up, investigate, and absolutely arrest someone who made a loud, direct, threat to murder another human being in public.
Not anywhere being discussed here. Chicago it may or may not get added to a dispatch call. Chances of it ever being followed up on are slim. At best you might get a patrol car rolling through a couple hours later to mark the ticket solved.
It’s a pretty common experience for anyone who is out and about using public transit in public spaces. They know nothing will be done to the suspect so what’s the point of bothering to respond at all?
I probably would not have called the police after this person if this happened to me, because I expect they would do nothing about it.