This is just the worst excuse of a simplified reason. Marco and Star didn't have that kind of relationship, during the whole show we barely see any progress other than "Muh hug"
Just one episode before the first season finale we had Marco so done with Star's shit that he actually asked him if she was capable of doing anything without the wand, and the expression the artist did on him was that of pure disgust over her.
Now about the "love" department, Marco and Star really had nothing between going on, other than a curse that was clearly forcing him to get closer to each other, on Marco it even looks like it's eating his brains so he is as dumb as star so he can like her, battle for mewni is a good example, he even didn't try to hide he disliked Mewni but then when season 3 comes through it suddenly "is of his liking".
I honestly think someone in there was actually trying to "lore it up" with the curse of the bloodmoon but since the rest of the guys (Daron included) didn't give a fuck about the lore they just did what they pleased and forgot about character coherence, development and fleshing which went off the roof.
Marco in season 3 just feels like a shadow of what he was becomming during season 2, it feels as if it's some other character that looks like him and is acting like him, but clearly is not him.