Because the git history will still keep it completely to be able to restore it once you go to a commit in which it wasn’t deleted.
And you cannot tell git to completely forget about it as that requires a rewrite of history and this changes all commit hashes which are touched (and any future ones).
They most likely did in a later commit. However the commit adding it can not be removed as OP said. So in order for git to be able to work properly the file is still in the repository in case someone wants to check out a commit where it still was present.
Why can’t you just remove the file?
Because the git history will still keep it completely to be able to restore it once you go to a commit in which it wasn’t deleted.
And you cannot tell git to completely forget about it as that requires a rewrite of history and this changes all commit hashes which are touched (and any future ones).
They most likely did in a later commit. However the commit adding it can not be removed as OP said. So in order for git to be able to work properly the file is still in the repository in case someone wants to check out a commit where it still was present.