Lol ikr? Maybe it's because they act different. Like Alya said in the volpina episode,"with a cat suit and cat ears Dont you think adrien looks a bit like chat noir?" And marinette replies," What no way! Adrien is tons more ligit then chat noir!" So maybe she thinks it isn't him because adrien is kind and a gentleman and chat noir is flirty and dorky. (My little dork) And Adrien probably doesn't think marinette is ladybug because marinette always acts awkward around him and ladybug is so brave. That's my theroy
How it makes sense: Adrien's hair is a less tame and styled different His eyes change to more cat like and a more neon green His voice changes a bit to be more eased and joking His movements and habits change a bit too He acts completely different So when you think about it Marinette not realizing it makes sense because there are a ton of boys in France with blonde hair and green eyes that style their hair differently than Adrien, while Adrien (as far as she knows) has a constant hairstyle. Coupling that with the fact he doesn't act anything like Adrien (except for a few heart breaking moments) and that his voice and habits change a bit it totally makes sense. You know if you or I went to their school and saw them on a day to day we would of suspected right off the bat because it's obvious but if we didn't and we only knew Adrien from posters would we put it together? Be honest.
But Marinette is a totally different yet completely similar case. Her hair is the exact same Her eyes are the exact same Her voice is more confident and sure Her movements change but she keeps most of her habits She acts completely different. So it is possible that he would figure it out because her hairstyle and eyes stay the same but it is plausible he wouldn't. Besides from when he first met her he's only heard her talk all flustered and unsure around him and Ladybug sounded like her at the beginning and you would think oh he would realize right? WRONG! When he first heard LB speak she was unsure and when he first heard Mari speak she was sure. They eventually transitioned into each other so it makes a bit of sense. Mari is clumsy around him and slouches a lot so it makes sense that he wouldn't match her with LB who always stands up tall and nearly never trips or falls. This one would be easier to tell if we attended there school but if we didn't then we probably wouldn't know her except possibly from her family's bakery.
Though I have to say there are moments where it's too obvious. Like when Cat Noir and Adrien both have an allergic reaction to feathers in Mr. Pidgeon , or when Mari totally owns her LB side in Darkblade or when IN THE EXACT SAME EPISODE ADRIEN JUST MAGICALLY SHOWS UP TO CHLOE'S CAMPAIGN PARTY.... Ahem sorry.. or when in The Bubbler Mari is the ONLY ONE of Adrien's friends that isn't at the party or when Cat Noir could probably totally see it was Mari in Anti bug....
But ultimately it's the Clark Kent effect. You wouldn't expect a well-mannered model to me a flirty, pun loving, Cat themed superhero who falls on his face at least once every time he shows up or a clumsy,shy girl who works at a bakery to be a confident, amazing, sassy, Lady bug themed super hero who can defeat a super villan in under five minutes so.... you don't see it. Just like you don't expect a clumsy, goofy, old fashioned, sweater-vest wearing, guy who "needs" glasses to be a Alien from another planet who can do anything impossible and probably kill you in under a second. So you don't see it.
I probably over did it but I hope it answers your question!