Mastered pirates flock to guilds depending on what kind of person they are. Many high leveled pirates at that stage in the game find it entertaining to play the game with pirates that share the same interests as they, hence you see why some guilds that focus on privateering, PvP, role-play, and so forth are often flocked with Level 50's.
It may also be the case that your friend's guild is closed off to high level pirates only, perhaps as an attempt to keep the atmosphere free from lower level pirates who may constantly need help, or not be able to partake in guild events that you are capable of.
And then on the other hand, you may just have those pirates come your way for the sake of having fun. PNC is home to over twenty mastered pirates (which is 1/10th of the guild) and we don't "aim" to recruit them. Whether they've leveled in the guild with us, or simply came to us because they wanted a guild with our personality, is up to them.
Each and every player has their own different view of the game and personality, regardless of level. See it as your initiative to make your guild as attractive to as many pirates as possible - and who knows? As pirates flock to you you may end up with a few high levels in there
