While it seemed like the obvious route, here's why the original Resident Evil movie didn't actually adapt any of the video games. Since the Resident Evil games debuted in 1996, the franchise has been spin-off in just about every medium. The first movie arrived in 2002 and spawned five direct sequels and a 2021 reboot debuted Resident Evil: Welcome To Raccoon City. The property has also received three animated films, a Netflix animated series and an upcoming live-action Resident Evil show starring Lance Reddick as Wesker.
Each part of the Resident Evil universe feels like its own unique beast, with the Milla Jovovich movies having few links to the games outside of some characters and creatures. Welcome To Raccoon City tried to appease fans of the games who were left cold by past movies by adapting the first and second games together. Unfortunately, this resulted in a crowded narrative with too many characters and subplots; the reboot's modest budget also resulted in some subpar CGI.
Famously, the late, great George A. Romero nearly helmed the original Resident Evil film. The rights to the first game were purchased by a company named Constantin Film, who felt a Resident Evil movie adaptation should be relatively simple to put together. However, according to a Fangoria #211 article titled "Resident Evil: Girls, Guns and Ghouls," (via Resident Evil Wiki) company head Robert Kulzer felt the release of the second game in 1998 would make a straight adaptation of the original title look "... really dated and boring." After developing scripts with screenwriter Alan McElroy and Romero, Constantin nearly let the option to produce Resident Evil expire before they connected with director Paul W.S. Anderson.
Having previously helmed Mortal Kombat and Event Horizon, Anderson seemed like the natural pick. He had also become a huge fan of the games and had tried to get the rights before learning Constantin had acquired them already. Anderson later developed a screenplay titled Undead, which he describes as "... very much a ripoff of Resident Evil," with an aim to either make it an original project or tweak it to be a Resident Evil movie (which won't be connected to the show) if the Romero version didn't happen.
After Romero exited Resident Evil, Anderson signed on to the movie and Undead - which again, borrowed elements from the games without being a straight adaption - was reworked. By the time the film went into production, the first three games and Dreamcast sequel Code: Veronica had already been released, so it's odd Constantin felt it would be "boring" to adapt the original or any of the others. Considering the success the film series met with general audiences, however, maybe Kulzer had a point.