Every game is “basically an X with a Y”, that’s not a reason for basically anything ever.
We can sit here and invent lore reasons all day for why the universes is setup like it is. Maybe ships just don’t have the power, maybe they don’t have the space, maybe they don’t have the supporting systems, and maybe it’s a combination of these.
It’s a game, if you’re not willing to suspend your disbelief and invest yourself in the game’s universe then you’re going to have a pretty much never ending list of these “consistency errors” in pretty much any game.
This is because if you actually understand how Eve is balanced and how the balance between small and large ships in Eve works you’d realize instantly, with zero explanation, why adding effective ‘secondaries’ to a Battleship in Eve would be so blindly and obviously over powered.
The short answer here is that large ships have more of basically everything than smaller ships. More HP, more range, more damage, more slots for utility and tank, more fitting space to fill those slots, ect. The only thing larger ships don’t have is speed, but that’s not worth much in Eve except for running away if you can’t get in and get under a larger ship’s guns in some way.
Adding secondary weapons that are effective against small ships destroys this and therefore destroys the balance between ship classes.