@jens yeah I've no problem with "hey, your character spent a month studying X? Bump that stat or add that skill." In fact, that could be great for the character's story arc BUT ... "oh, you killed some baddies / solved the mystery so suddenly you're more buff / intelligent / have a new superpower ... just ⁉️ it makes no sense.
Also, in traditional games we have fun playing AND we level up NOT we have fun playing BECAUSE we level up.