At this point, it is but it isn't.

They're basically like a James Bond movie or The Fast and the Furious series. Past events have an impact on current events and there's a rotating cast of core characters who make up the main gang of the protagonists, but the games also do a good job of being self-contained, standalone adventures which explain the who, what, and when all of the characters in a given game matter to new players so they don't miss out.

The only real exception to this is Resident Evil 6, which was designed as a huge, Avengers-like mega game which brought back the entire cast for a huge spectacle of continuity, ruined by the fact that it's one of the worst in the series, and easily the worst game which is an official, numbered sequel instead of being a spin-off.

If you've got time, they're still worth playing. I'd even recommend giving the original originals and not the remakes a try if you think you won't be bothered by the graphics, because there are some things that the originals do better than the remakes.