True. But Karloff did return to the Frankenstein series as a mad scientist three movies later, while supporting actors like Lionel Atwell and Dwight Frye played multiple roles over the course of the Frankenstein films, sometimes one after another.
Indeed, Lionel Atwill was in
five consecutive Frankenstein movies in five different roles -- heroic Inspector Krogh in
Son, evil Dr. Bohmer in
Ghost, Mayor in
Meets the Wolf Man, and two other police inspectors in
House of Frankenstein and
House of Dracula.
Toho's Godzilla and other kaiju films did much the same thing -- casting the same actors over and over again in different roles, sometimes just the same essential character with a name change. It's pretty strange -- sometimes you have to wonder why they don't just keep the character name. A particularly bizarre example is in
Frankenstein's Monsters: Sanda vs. Gaira, aka
The War of the Gargantuas. In its plot, it's unambiguously a direct sequel to the events of
Frankenstein vs. Baragon, aka
Frankenstein Conquers the World (though the connection was glossed over in the English dub), with what are recognizably meant to be the same three lead characters, but all three are renamed, two are recast, and their home base is retconned from Hiroshima to Kyoto.
And I guess Larry Talbot never noticed that Ygor (Bela Lugosi) looked a lot like the gypsy werewolf who bit him in the first place (also played by Bela).
Actually Ygor was in the previous two films,
Son and
Ghost. At the end of
Ghost, Lugosi's Ygor had his brain transplanted into Chaney's Monster -- and then in
Meets, Chaney's Talbot/Wolf Man met Lugosi as the Monster. (He was still supposed to have Ygor's mind and voice, and be blind from the previous film, hence the famous hands-forward posture. But that all got cut out by executive fiat.) Then came
House of Frankenstein and
House of Dracula with Chaney as Talbot, Glenn Strange as the Monster, and John Carradine as Dracula. Lugosi would finally return to the role of Dracula opposite Chaney and Strange in
Abbott and Costello Meet Frankenstein.
Can you imagine how fans would react today if, say, Jeff Bridges played the villain in the first IRON MAN movie, then popped up again as the Danish scientist as the THOR movies, and then as Howard Stark in IRON MAN IV?
They had enough trouble with Alfre Woodard playing different roles in
Civil War and
Luke Cage. Though there have been others -- Tony Curran had different roles in
Thor: The Dark World and
Daredevil, I think it was, and Enver Gjokaj was a cop in
The Avengers and a '40s SSR agent in
Agent Carter. Plus there's Stan Lee, of course, and dozens of bit players.