I think it's because they act different when their in other forms then when their not. And also, Marinette and Adrien don't know the other's secret identity's. When people ship marichat, they want Chat Noir to rendez-vous amoureux, date Marinette without them necessarily knowing that she is ladybug and he is Adrien. And with Ladynoir, they want Ladybug and Chat Noir to rendez-vous amoureux, date without knowing their regular forms. And kinda the same with Marinette x Adrain and the others. It is kinda confusing though. Personally I just ship them as one ship, not like 4
You’re technically right but, they have almost completely different personalities as Ladybug and Adrien. Ladybug is confident and fearless while Marinette is kinda cowardly and super clumsy. Adrien is a gentleman and a kindhearted soul while Cat Noir is this to he is plus flirtatious and loose. Ya know? Btw I ship Marichat!