If a fighter spends a lot of their time making perception checks, actively looking out for things, fighting off mind-affecting spells and learning what sort of things work and what doesn't, its up to the player to justify why their perceptiveness, willpower and common sense haven't improved.
Likewise if a rogue spends a lot of time climbing walls, running around, and yomping a pack full of rations, camping equipment, weapons and loot up hill and down dale, it is up to the player to explain why they still still have their Str-8 couch-potato physique.
If a wizard has spent a lot of their time watching the fighter kill orcs with a sword, and being attacked by orcs with swords, then they probably still don't understand the nuances of using a sword without training and practice, but they are probably much better at not getting killed by swords than they were when they started out.