The Resident Evil movies often introduced major characters from the games, but here's why the majority of them didn't end up getting killed. In 2002 Paul W.S. Anderson wrote and directed the original Resident Evil movie, which served as an incredibly loose adaptation of the first game. It starred Milla Jovovich as Alice, a character created specifically for the film, and only borrowed certain elements from the games such as the Umbrella Corporation, Raccoon City or creatures like The Licker.
It wasn't until the sequels that characters from the video games started appearing and playing major roles. The Resident Evil movies can have a confusing timeline, though, thanks to a habit of soft rebooting the story between entries, which often leaves certain heroes' fates unresolved. The fifth movie Retribution ended with Alice standing on top of The White House with game figures like Jill Valentine, Ada Wong, and Leon Kennedy, ready to fight the last battle for mankind. Yet in Resident Evil: The Final Chapter, this battle has happened offscreen and Alice was seemingly the sole survivor. While references are made to series villain Wesker betraying Alice and her comrades, bizarrely, the movie doesn't name her allies or even confirm their fates.
The reasoning comes from something viewers may have suspected but was never officially confirmed; a mandate from Capcom preventing the killing off of game characters. It's not a secret now, as Milla Jovovich herself casually confirmed it during a 2017 Collider interview. The actress stated of the decision to bring in new characters that "...we need a body count and it’s important that we have people that are going to die, and I am not going to die and you know characters from the game can’t die because they are owned by Capcom and we’re not allowed to kill any of them off." This mandate led to strange moments in the final entry like Alice reuniting with Claire Redfield from previous movies, all while her brother Chris' fate after the fourth film Afterlife is left unresolved and isn't even commented on by his own sister. By the time The Final Chapter's end credits rolled, the fates of Chris, Jill, Leon, and Ada are still left in the air, and are unlikely to ever be resolved.
The fact the game characters disappear rather than carry over to new entries is also explained by the other part of Jovovich's quote, where each new entry needed a high body count. Original characters had to be added so that they could be cannon fodder, so if the unkillable game characters stuck around then the cast of the later films would be unwieldy. Since figures like Chris or Ada can't be killed off, they simply vanish and are never referred to ever again. If this mandate will affect the Resident Evil reboot isn't clear yet.
It should be noted, however, the movies did make exceptions to this rule on occasion. Both Carlos Olivera and Barry Burton died in the third and fifth entries respectively, but they wouldn't be considered headliners like Jill or Leon, while Wesker himself also perished in the final movie. What seemed like a bizarre inconsistency or sloppy writing actually came out of the needs of both Capcom as a brand and the needs of the films themselves, and it ended up making the Resident Evil movies a unique beast distinct from the games they adapted.