Why is materia capped/unusable on gear?

[Nero]

I understand this. But what I don't understand is why there is a cap, and why bother with materia if the items you get will already be capped in the areas that matter? Is materia to be considered just another way to make an item HQ then?

To prevent people from making pure op-items like this:



There has to be a cap, or you will have a system which is not balanced at all.