In-universe, when they learn that "Jay Garrick" is actually Hunter Zolomon, a psychopath from Earth 2, they don't know about the existence of a real Jay Garrick and think that name is just a random stage name for his Earth-2 Flash persona, and not the name of a real person (the Earth-3's Flash). For them, Jay Garrick=Hunter Zolomon=Zoom.
Since they are used to call him that way, they have no reason to change. The real name of the famous actor Michael Caine is Maurice Micklewhite Jr.. Knowing this, will you change the way you call him?
Out-of-universe, it is for convenience for the audience.