Sounds like you don't play with good (not great) lefty players, do you?
I"ve played with lefties since 1975. NOT ALL, but the majority are very talented, but don't take it nearly as seriously as their rightie brethen.
I can name TWO lefties who DO take it seriously, but have limited stroke production potential (weak serves, or lacking in some aspects of the game.
I can probably name well over 20 lefties who had the potential and EVERY shot to get to the next level (compared to their peers) but never did, because of head games, lack of real motivation, or lack of interest.
For sure, Connors and McEnroe took it seriously and maximised their potential.
I can think of dozens of lefties who had better physical skills, stroke production, speed, power, who didn't come close to that level.