you actually just stated the reason yourself
players that are committed to lee sin can and will learn how to master him.
According to champion.gg for lee, he reaches 50% win rate at only 15 games, not 50 whereas at 50 games, his win rate is at 53% . As per how confusing statistics are, why does champion.gg's statistics list his win rate going DOWN after 50 wins?
as for lee being in every game, he has close to a 28% playrate, meaning 1 in 4 matches will include him in platinum and above. If we take a look at lee sin's statistics, the higher up you go, the higher lee's win rate is, even passing 60%
lee is a popular champion, but lets take a look at the less popular azir. Azir has a 2.49% pick rate, meaning he will appear in 1/50 games. His win rate base is roughly 42%
even after 125 games, his win rate is still only 46%
i'll take a look at another popular champion, Zed.
Zed has an average 50% win rate beginning at only 5 games played.
zed is supposed to be a mechanically intensive champion with lots of strategic play involved and the ability to easily make a mistake and ruin his setup
if statistics were really viable in league, Aatrox will be receiving a nerf soon because out of almost 600 games played this season (apparently only played by dedicated mains), Aatrox has a 100% win rate at only 15 games played and maintains that stat to 50 games played then rapidly drops to a 3% win rate at a total of 125 games played
the consistent pattern with popular champions? easy to learn and easy to win with