Lack of discipline and Lack of inclusivity.Any power creep is due to designer discipline, and independent of the actual rules.
The power creep happens because the designers both refuse to broaden the game seriously AND do not discipline themselves when dealing with the old.
There is a reason why power creep in TTRPGs is usual in the original or traditional chassises and styles.
It's not modern Gacha Power Creep where the new ideas wholly replace the old.
TTRPG power creep is usually designers not stopping themselves from the old favorite dog new toys and that dog mixing and matching.