@Boag
Even from an everyman perspective the logic doesn't check out.
You can't exercise your muscles to superhuman strength and learn better magic just via pushups and the occasional drink of water.
He refuses to listen to the literal god trying to explain to him how the physics work in an entirely unknown universe to him.
Better armour being better protection is not game logic at all. It's regular logic and he can't even follow that.
This is assuming that he's not native to a weird dimension where his behavior is perfectly normal.
He's supposed to be cautious but all I see is the idiot who complains about not knowing something that's in the instruction manual with the goddess being the instruction manual.
That's presumably why cautious is listed as a weakness on his character sheet. He's too dumb to actually be cautious in an advantageous way like the demon lord was.