More on why eating fish was never common even in real life.
Think like this, there's a random fishing village, first of all selling to small villages is not worth, so let's say the closest big city is 10 days away, even moving small quantity of fish a merchant would need to buy a
lot of ice to preserve it, which means not only fish price would go through the roof, but they probably would need to move even more fish to make it worth at all, making the caravan even larger, which makes it an ever bigger target for bandits, which mean the merchant needs to hire even more protection.
Isekai Walking has magic so it simplify some of those things, hire a mage to use ice spells for example, magic bag doesn't mix items though, so you can't put a lot of ice to preserve the fish on it, so large caravan is still a must, but then there's monsters too, which means you need good adventurers to protect you, which isn't cheap.
Only way to make fish accessible at all is with spatial magic, which is rare as fuck, don't remember any mention of magic bag that stops time, but even regular magic bag is rare as fuck, so moving a lot of fish by abusing inventory that preserves it isn't possible either.
In short, due to the logistics eating fish in the past was literally only a thing for the rich and powerful, only way to eat fish as a filthy peasant is if you live near a river, lake or in a port, which then fishihg is inevitable.
People underestimate just how much industrialization made things accessible for general public, cause in the past you would eat whatever you had at hand.
To think a different isekai would teach me fish logistics