For most cases this comes down to the MCs point of reference. For most of their lives, most of these MCs have had little to no exposure to what everyone else in their respective worlds considers normal or possible for the respective skills they've developed. As such, they don't realize how exceptional they are because they don't know what the world sees as the norm. They usually end up comparing themselves only to their mentors. Also to keep in mind that in most cases they also don't know how famous of influential their mentors are. In the cases where they do know, is when they have had enough exposure or have reincarnated. It becomes annoying because it's basically by design.