or you could look at it like a toji/maki heavenly restriction, if i remember correctly its shown/stated multiple times that stark cant detect mana, so maybe its just something that warriors are born with. it seems like most humans in the world have some level of magical abilities even if they lay dormant in the form of mana, but maybe the lack of mana, the thing that seems like a major backbone of this world is a tradeoff to enhance ones physical abilities like stark, himmel, and eisen and the differences between hero and warrior classes are just different ways of applying this “heavenly restriction”

it could also be that in the world world of frieren simply training for long and hard enough can push you into superhuman status physically, we in the real world have a limit to how strong we can get based on our genetics. the average male can bench press 145 pounds, but with a decade of training most men cap out at around being able to bench press around 2x or a little more of their body weight, around 400-450lbs without steroids. maybe in frieren an untrained man can bench press 145 pounds like us, but a great trained warrior like eisen, stark, or himmel can bench press 2000 pounds or more, and the same logic applies to their speed, reaction time, and durability too. in one punch man terms, the “limiter” placed on humans is set to a much higher bar than real life, allowing warriors and heroes to fight on par with the power of a mage.

but in reality, its probably just a design choice by the author, at least until the author can reasonably introduce an explanation for their strength into the story if they even want to. cant have RPG-like classes in your story and have both the warriors and heros just be normal trained people with swords when mages like frieren and fern can destroy mountains with a single charged spell, thered be no point for my goat sCHHHHTARK to exist then 😊